diff --git a/fluent-package/apt/systemd-test/install-newly.sh b/fluent-package/apt/systemd-test/install-newly.sh
index 37216cef9..0779de975 100755
--- a/fluent-package/apt/systemd-test/install-newly.sh
+++ b/fluent-package/apt/systemd-test/install-newly.sh
@@ -28,6 +28,14 @@ sudo fluent-gem install fluent-plugin-concat
 /opt/fluent/bin/fluent-diagtool -t fluentd -o /tmp
 test $(find /tmp/ -name gem_local_list.output | xargs cat) = "fluent-plugin-concat"
 
+# Test: Guard duplicated instance
+if [ "$1" = "local" ]; then
+    # FIXME: until guard feature was released, skip v5 and lts.
+    (! sudo /usr/sbin/fluentd)
+    (! sudo /usr/sbin/fluentd -v)
+    sudo /usr/sbin/fluentd --dry-run
+fi
+
 sudo apt remove -y fluent-package
 
 case ${code_name} in
diff --git a/fluent-package/apt/systemd-test/update-from-v4.sh b/fluent-package/apt/systemd-test/update-from-v4.sh
index 36a2c5fcf..16926a770 100755
--- a/fluent-package/apt/systemd-test/update-from-v4.sh
+++ b/fluent-package/apt/systemd-test/update-from-v4.sh
@@ -84,6 +84,12 @@ sudo fluent-gem install fluent-plugin-concat
 /opt/fluent/bin/fluent-diagtool -t fluentd -o /tmp
 test $(find /tmp/ -name gem_local_list.output | xargs cat) = "fluent-plugin-concat"
 
+# Test: Guard duplicated instance
+(! sudo /usr/sbin/fluentd)
+(! sudo /usr/sbin/td-agent)
+(! sudo /usr/sbin/fluentd -v)
+sudo /usr/sbin/fluentd --dry-run
+
 # Uninstall
 sudo apt remove -y fluent-package
 (! systemctl status --no-pager td-agent)
diff --git a/fluent-package/apt/systemd-test/update-to-next-version-with-backward-compat-for-v4.sh b/fluent-package/apt/systemd-test/update-to-next-version-with-backward-compat-for-v4.sh
index e8cf5aa91..30f42bf92 100755
--- a/fluent-package/apt/systemd-test/update-to-next-version-with-backward-compat-for-v4.sh
+++ b/fluent-package/apt/systemd-test/update-to-next-version-with-backward-compat-for-v4.sh
@@ -80,6 +80,12 @@ sleep 3
 test -e /var/log/fluent/fluentd.log
 (! grep -e '\[error\]' -e '\[fatal\]' /var/log/fluent/fluentd.log)
 
+# Test: Guard duplicated instance
+(! sudo /usr/sbin/fluentd)
+(! sudo /usr/sbin/td-agent)
+(! sudo /usr/sbin/fluentd -v)
+sudo /usr/sbin/fluentd --dry-run
+
 # Uninstall
 sudo apt remove -y fluent-package
 (! systemctl status --no-pager td-agent)
diff --git a/fluent-package/apt/systemd-test/update-to-next-version.sh b/fluent-package/apt/systemd-test/update-to-next-version.sh
index 31f5410c1..bda33fc68 100755
--- a/fluent-package/apt/systemd-test/update-to-next-version.sh
+++ b/fluent-package/apt/systemd-test/update-to-next-version.sh
@@ -49,6 +49,12 @@ sleep 3
 test -e /var/log/fluent/fluentd.log
 (! grep -q -e '\[warn\]' -e '\[error\]' -e '\[fatal\]' /var/log/fluent/fluentd.log)
 
+# Test: Guard duplicated instance
+(! sudo /usr/sbin/fluentd)
+(! sudo /usr/sbin/td-agent)
+(! sudo /usr/sbin/fluentd -v)
+sudo /usr/sbin/fluentd --dry-run
+
 # Uninstall
 sudo apt remove -y fluent-package
 (! systemctl status --no-pager td-agent)
diff --git a/fluent-package/templates/usr/sbin/fluentd.erb b/fluent-package/templates/usr/sbin/fluentd.erb
index 8e3833a42..d261062c0 100755
--- a/fluent-package/templates/usr/sbin/fluentd.erb
+++ b/fluent-package/templates/usr/sbin/fluentd.erb
@@ -12,4 +12,39 @@ if ARGV.include?("--version")
   puts "fluent-package #{PACKAGE_VERSION} fluentd #{Fluent::VERSION} (#{FLUENTD_REVISION})"
   exit 0
 end
+
+class DuplicateInstanceChecker
+  def running_fluentd?
+    IO.popen(["systemctl", "is-active", "fluentd"]) do |_io|
+      return true if _io.readlines.last.chomp == "active"
+    end
+    false
+  end
+
+  def prevent_duplicate_launch?
+    unless running_fluentd?
+      return false
+    end
+    true
+  end
+end
+
+if RUBY_PLATFORM =~ /linux/
+  checker = DuplicateInstanceChecker.new
+  if checker.prevent_duplicate_launch?
+    unless ARGV.include?("-c") or ARGV.include?("--config") or ARGV.include?("--dry-run")
+      puts("Error: Can't start duplicate Fluentd instance with the default config.")
+      if ARGV.include?("-v")
+	puts("To take the version, please use '--version', not '-v' ('--verbose').")
+      end
+      puts <<EOS
+To start Fluentd, please do one of the following:
+(Caution: Please be careful not to start multiple instances with the same config.)
+- Stop the Fluentd service 'fluentd'.
+- Specify the config path explicitly by '-c' ('--config').
+EOS
+      exit 2
+    end
+  end
+end
 load "<%= install_path %>/bin/fluentd"
diff --git a/fluent-package/yum/systemd-test/install-newly.sh b/fluent-package/yum/systemd-test/install-newly.sh
index 70e6c6168..46cf8402c 100755
--- a/fluent-package/yum/systemd-test/install-newly.sh
+++ b/fluent-package/yum/systemd-test/install-newly.sh
@@ -51,6 +51,10 @@ if [ $1 = "local" ]; then
     sudo $DNF install -y tar findutils
     sudo /opt/fluent/bin/fluent-diagtool -t fluentd -o /tmp
     test $(find /tmp/ -name gem_local_list.output | xargs cat) = "fluent-plugin-concat"
+    # FIXME: until guard feature was released, skip v5 and lts.
+    (! sudo /usr/sbin/fluentd)
+    (! sudo /usr/sbin/fluentd -v)
+    sudo /usr/sbin/fluentd --dry-run
 fi
 
 sudo $DNF remove -y fluent-package
diff --git a/fluent-package/yum/systemd-test/update-from-v4.sh b/fluent-package/yum/systemd-test/update-from-v4.sh
index 480e12df6..1593893e7 100755
--- a/fluent-package/yum/systemd-test/update-from-v4.sh
+++ b/fluent-package/yum/systemd-test/update-from-v4.sh
@@ -96,6 +96,12 @@ sudo fluent-gem install fluent-plugin-concat
 sudo /opt/fluent/bin/fluent-diagtool -t fluentd -o /tmp
 test $(find /tmp/ -name gem_local_list.output | xargs cat) = "fluent-plugin-concat"
 
+# Test: Guard duplicated instance
+(! sudo /usr/sbin/fluentd)
+(! sudo /usr/sbin/td-agent)
+(! sudo /usr/sbin/fluentd -v)
+sudo /usr/sbin/fluentd --dry-run
+
 # Uninstall
 sudo $DNF remove -y fluent-package
 sudo systemctl daemon-reload
diff --git a/fluent-package/yum/systemd-test/update-to-next-version-with-backward-compat-for-v4.sh b/fluent-package/yum/systemd-test/update-to-next-version-with-backward-compat-for-v4.sh
index 25b89a062..f18dabbb2 100755
--- a/fluent-package/yum/systemd-test/update-to-next-version-with-backward-compat-for-v4.sh
+++ b/fluent-package/yum/systemd-test/update-to-next-version-with-backward-compat-for-v4.sh
@@ -115,6 +115,12 @@ sudo fluent-gem install fluent-plugin-concat
 sudo /opt/fluent/bin/fluent-diagtool -t fluentd -o /tmp
 test $(find /tmp/ -name gem_local_list.output | xargs cat) = "fluent-plugin-concat"
 
+# Test: Guard duplicated instance
+(! sudo /usr/sbin/fluentd)
+(! sudo /usr/sbin/td-agent)
+(! sudo /usr/sbin/fluentd -v)
+sudo /usr/sbin/fluentd --dry-run
+
 # Uninstall
 sudo $DNF remove -y fluent-package
 (! systemctl status --no-pager td-agent)
diff --git a/fluent-package/yum/systemd-test/update-to-next-version.sh b/fluent-package/yum/systemd-test/update-to-next-version.sh
index 83c5573ae..10dd7ce57 100755
--- a/fluent-package/yum/systemd-test/update-to-next-version.sh
+++ b/fluent-package/yum/systemd-test/update-to-next-version.sh
@@ -73,6 +73,12 @@ sudo fluent-gem install fluent-plugin-concat
 sudo /opt/fluent/bin/fluent-diagtool -t fluentd -o /tmp
 test $(find /tmp/ -name gem_local_list.output | xargs cat) = "fluent-plugin-concat"
 
+# Test: Guard duplicated instance
+(! sudo /usr/sbin/fluentd)
+(! sudo /usr/sbin/td-agent)
+(! sudo /usr/sbin/fluentd -v)
+sudo /usr/sbin/fluentd --dry-run
+
 # Uninstall
 sudo $DNF remove -y fluent-package
 sudo systemctl daemon-reload