diff --git a/lib/fluent/system_config.rb b/lib/fluent/system_config.rb
index 2b82835f94..30e1cafdf1 100644
--- a/lib/fluent/system_config.rb
+++ b/lib/fluent/system_config.rb
@@ -98,6 +98,13 @@ def attach(supervisor)
             next # doesn't exist in command line options
           when :emit_error_log_interval
             system.emit_error_log_interval = @suppress_interval if @suppress_interval
+          when :log_level
+            ll_value = instance_variable_get("@log_level")
+            # info level can't be specified via command line option.
+            # log_level is info here, it is default value and <system>'s log_level should be applied if exists.
+            if ll_value != Fluent::Log::LEVEL_INFO
+              system.log_level = ll_value
+            end
           else
             next unless instance_variable_defined?("@#{param}")
             supervisor_value = instance_variable_get("@#{param}")
diff --git a/test/config/test_system_config.rb b/test/config/test_system_config.rb
index ba7feb6317..3fa31db320 100644
--- a/test/config/test_system_config.rb
+++ b/test/config/test_system_config.rb
@@ -13,11 +13,13 @@ def initalize
   end
 
   class FakeSupervisor
+    attr_writer :log_level
+
     def initialize
       @workers = nil
       @root_dir = nil
       @log = FakeLoggerInitializer.new
-      @log_level = nil
+      @log_level = Fluent::Log::LEVEL_INFO
       @suppress_interval = nil
       @suppress_config_dump = nil
       @suppress_repeated_stacktrace = nil
@@ -55,7 +57,7 @@ def parse_text(text)
       assert_nil(sc.without_source)
       assert_equal(1, s.instance_variable_get(:@workers))
       assert_nil(s.instance_variable_get(:@root_dir))
-      assert_nil(s.instance_variable_get(:@log_level))
+      assert_equal(Fluent::Log::LEVEL_INFO, s.instance_variable_get(:@log_level))
       assert_nil(s.instance_variable_get(:@suppress_repeated_stacktrace))
       assert_nil(s.instance_variable_get(:@emit_error_log_interval))
       assert_nil(s.instance_variable_get(:@suppress_config_dump))
@@ -99,7 +101,7 @@ def parse_text(text)
       sc.apply(s)
       assert_equal(1, s.instance_variable_get(:@workers))
       assert_nil(s.instance_variable_get(:@root_dir))
-      assert_nil(s.instance_variable_get(:@log_level))
+      assert_equal(Fluent::Log::LEVEL_INFO, s.instance_variable_get(:@log_level))
       assert_nil(s.instance_variable_get(:@suppress_repeated_stacktrace))
       assert_nil(s.instance_variable_get(:@emit_error_log_interval))
       assert_nil(s.instance_variable_get(:@suppress_config_dump))
@@ -109,16 +111,43 @@ def parse_text(text)
       assert_nil(s.instance_variable_get(:@dir_permission))
     end
 
-    test 'log_level' do
+    data('trace' => Fluent::Log::LEVEL_TRACE,
+         'debug' => Fluent::Log::LEVEL_DEBUG,
+         'info' => Fluent::Log::LEVEL_INFO,
+         'warn' => Fluent::Log::LEVEL_WARN,
+         'error' => Fluent::Log::LEVEL_ERROR,
+         'fatal' => Fluent::Log::LEVEL_FATAL)
+    test 'log_level is applied when log_level related command line option is not passed' do |level|
+      conf = parse_text(<<-EOS)
+        <system>
+          log_level #{Fluent::Log::LEVEL_TEXT[level]}
+        </system>
+      EOS
+      s = FakeSupervisor.new
+      sc = Fluent::SystemConfig.new(conf)
+      sc.attach(s)
+      sc.apply(s)
+      assert_equal(level, s.instance_variable_get("@log").level)
+    end
+
+    # info is removed because info level can't be specified via command line
+    data('trace' => Fluent::Log::LEVEL_TRACE,
+         'debug' => Fluent::Log::LEVEL_DEBUG,
+         'warn' => Fluent::Log::LEVEL_WARN,
+         'error' => Fluent::Log::LEVEL_ERROR,
+         'fatal' => Fluent::Log::LEVEL_FATAL)
+    test 'log_level is ignored when log_level related command line option is passed' do |level|
       conf = parse_text(<<-EOS)
         <system>
-          log_level warn
+          log_level info
         </system>
       EOS
       s = FakeSupervisor.new
+      s.log_level = level
       sc = Fluent::SystemConfig.new(conf)
+      sc.attach(s)
       sc.apply(s)
-      assert_equal(Fluent::Log::LEVEL_WARN, s.instance_variable_get("@log").level)
+      assert_equal(level, s.instance_variable_get("@log").level)
     end
 
     test 'process global overridable variables' do
@@ -130,6 +159,7 @@ def parse_text(text)
       EOS
       s = FakeSupervisor.new
       sc = Fluent::SystemConfig.new(conf)
+      sc.attach(s)
       sc.apply(s)
       assert_equal(0655, s.instance_variable_get(:@file_permission))
       assert_equal(0765, s.instance_variable_get(:@dir_permission))