diff --git a/lib/fluent/plugin/out_file.rb b/lib/fluent/plugin/out_file.rb
index a5bed3ea61..13f6c695cf 100644
--- a/lib/fluent/plugin/out_file.rb
+++ b/lib/fluent/plugin/out_file.rb
@@ -117,7 +117,17 @@ def configure(conf)
       configured_time_slice_format = conf['time_slice_format']
 
       if conf.elements(name: 'buffer').empty?
+        # no <buffer> section, default time chunk key and timekey (1d) will be used.
+        log.warn "default timekey interval (1d) will be used because of missing <buffer> section. To change the output frequency, please modify the timekey value"
+
         conf.add_element('buffer', 'time')
+      else
+        unless conf.elements(name: 'buffer').first.has_key?('timekey')
+          if conf.elements(name: 'buffer').first.arg != "[]"
+            # with <buffer> section (except <buffer []>), and no timekey
+            log.warn "default timekey interval (1d) will be used. To change the output frequency, please modify the timekey value"
+          end
+        end
       end
       buffer_conf = conf.elements(name: 'buffer').first
       # Fluent::PluginId#configure is not called yet, so we can't use #plugin_root_dir here.
diff --git a/test/command/test_fluentd.rb b/test/command/test_fluentd.rb
index b7b8095355..4eb295f0b7 100644
--- a/test/command/test_fluentd.rb
+++ b/test/command/test_fluentd.rb
@@ -1712,4 +1712,52 @@ def create_config_include_dir_configuration(config_path, config_dir, yaml_format
                          expected_warning_message)
     end
   end
+
+  sub_test_case "test suspicious timekey interval (1d) in out_file configuration" do
+    data('without buffer' => {buffer: nil, buffer_arg: nil, message: :missing_buffer},
+         'buffer' => {buffer: true, buffer_arg: '', message: :warning},
+         'buffer time' => {buffer: true, buffer_arg: 'time', message: :warning},
+         'buffer []' => {buffer: true, buffer_arg: '[]', message: nil})
+    test 'warn the default value of timekey (1d) is used as-is' do |data|
+      prefix = "default timekey interval (1d) will be used"
+      advice = "To change the output frequency, please modify the timekey value"
+      missing_warning = "#{prefix} because of missing <buffer> section. #{advice}"
+      warning = "#{prefix}. #{advice}"
+
+      conf_path = if data[:buffer]
+                    create_conf_file("warning.conf", <<~EOF)
+                      <system>
+                        config_include_dir ""
+                      </system>
+                      <match>
+                        @type file
+                        tag test
+                        path #{@tmp_dir}/test.log
+                        <buffer #{data[:buffer_arg]}>
+                          @type file
+                        </buffer>
+                      </match>
+                    EOF
+                  else
+                    create_conf_file("warning.conf", <<~EOF)
+                      <system>
+                        config_include_dir ""
+                      </system>
+                      <match>
+                        @type file
+                        tag test
+                        path #{@tmp_dir}/test.log
+                      </match>
+                    EOF
+      end
+      case data[:message]
+      when :missing_buffer
+        assert_log_matches(create_cmdline(conf_path, '--dry-run'), missing_warning, patterns_not_match: [warning])
+      when :warning
+        assert_log_matches(create_cmdline(conf_path, '--dry-run'), "warning", patterns_not_match: [missing_warning])
+      else
+        assert_log_matches(create_cmdline(conf_path, '--dry-run'), patterns_not_match: [warning, missing_warning])
+      end
+    end
+  end
 end