diff --git a/lib/fluent/log.rb b/lib/fluent/log.rb
index f9ecc82b43..f8175fd911 100644
--- a/lib/fluent/log.rb
+++ b/lib/fluent/log.rb
@@ -51,6 +51,8 @@ module TTYColor
     LOG_TYPES = [LOG_TYPE_SUPERVISOR, LOG_TYPE_WORKER0, LOG_TYPE_DEFAULT].freeze
     LOG_ROTATE_AGE = %w(daily weekly monthly)
 
+    IGNORE_SAME_LOG_MAX_CACHE_SIZE = 1000 # If need, make this an option of system config.
+
     def self.str_to_level(log_level_str)
       case log_level_str.downcase
       when "trace" then LEVEL_TRACE
@@ -477,6 +479,13 @@ def ignore_same_log?(time, message)
           false
         end
       else
+        if cached_log.size >= IGNORE_SAME_LOG_MAX_CACHE_SIZE
+          cached_log.reject! do |_, cached_time|
+            (time - cached_time) > @ignore_same_log_interval
+          end
+        end
+        # If the size is still over, we have no choice but to clear it.
+        cached_log.clear if cached_log.size >= IGNORE_SAME_LOG_MAX_CACHE_SIZE
         cached_log[message] = time
         false
       end
diff --git a/test/test_log.rb b/test/test_log.rb
index 8139162420..26dacb2adf 100644
--- a/test/test_log.rb
+++ b/test/test_log.rb
@@ -472,6 +472,43 @@ def test_different_message
       ]
       assert_equal(expected, log.out.logs)
     end
+
+    def test_reject_on_max_size
+      ignore_same_log_interval = 10
+
+      logger = Fluent::Log.new(
+        ServerEngine::DaemonLogger.new(@log_device, log_level: ServerEngine::DaemonLogger::INFO),
+        ignore_same_log_interval: ignore_same_log_interval,
+      )
+
+      # Output unique log every second.
+      Fluent::Log::IGNORE_SAME_LOG_MAX_CACHE_SIZE.times do |i|
+        logger.info "Test #{i}"
+        Timecop.freeze(@timestamp + i)
+      end
+      logger.info "Over max size!"
+
+      # The newest cache and the latest caches in `ignore_same_log_interval` should exist.
+      assert { Thread.current[:last_same_log].size == ignore_same_log_interval + 1 }
+    end
+
+    def test_clear_on_max_size
+      ignore_same_log_interval = 10
+
+      logger = Fluent::Log.new(
+        ServerEngine::DaemonLogger.new(@log_device, log_level: ServerEngine::DaemonLogger::INFO),
+        ignore_same_log_interval: ignore_same_log_interval,
+      )
+
+      # Output unique log at the same time.
+      Fluent::Log::IGNORE_SAME_LOG_MAX_CACHE_SIZE.times do |i|
+        logger.info "Test #{i}"
+      end
+      logger.info "Over max size!"
+
+      # Can't reject old logs, so all cache should be cleared and only the newest should exist.
+      assert { Thread.current[:last_same_log].size == 1 }
+    end
   end
 
   def test_dup