diff --git a/lib/fluent/plugin/out_remote_syslog.rb b/lib/fluent/plugin/out_remote_syslog.rb
index 2409525..4f51208 100644
--- a/lib/fluent/plugin/out_remote_syslog.rb
+++ b/lib/fluent/plugin/out_remote_syslog.rb
@@ -96,6 +96,8 @@ def write(chunk)
         program = extract_placeholders(@program, chunk.metadata)
         hostname = extract_placeholders(@hostname, chunk.metadata)
 
+        severity = SeverityMapper.map(severity)
+
         packet_options = {facility: facility, severity: severity, program: program}
         packet_options[:hostname] = hostname unless hostname.empty?
 
@@ -149,6 +151,22 @@ def create_sender(host, port)
         @senders << sender
         sender
       end
+
+      # Convert some Severity values that is not supported in `syslog_protocol` library:
+      #   https://github.com/eric/syslog_protocol
+      # We want to fix `syslog_protocol` itself, but it seems to be not maintained for a while.
+      # If the following PR is merged, we can remove this implementaion.
+      #   https://github.com/eric/syslog_protocol/pull/9
+      module SeverityMapper
+        DICT = {
+          # "warning" is not supported, but we should use it since "warn" is deprecated.
+          "warning" => "warn", 
+        }
+
+        def self.map(severity)
+          DICT[severity] || severity
+        end
+      end
     end
   end
 end
diff --git a/test/plugin/out_remote_syslog.rb b/test/plugin/out_remote_syslog.rb
index b6a779e..4ae18bf 100644
--- a/test/plugin/out_remote_syslog.rb
+++ b/test/plugin/out_remote_syslog.rb
@@ -81,4 +81,19 @@ def test_write_tcp
       d.feed("tag", Fluent::EventTime.now, {"message" => "foo"})
     end
   end
+
+  data("emerg", {in: "emerg", out: "emerg"})
+  data("alert", {in: "alert", out: "alert"})
+  data("crit", {in: "crit", out: "crit"})
+  data("err", {in: "err", out: "err"})
+  data("warn", {in: "warn", out: "warn"})
+  data("warning", {in: "warning", out: "warn"})
+  data("notice", {in: "notice", out: "notice"})
+  data("info", {in: "info", out: "info"})
+  data("debug", {in: "debug", out: "debug"})
+  data("wrong", {in: "wrong", out: "wrong"})
+  def test_severity_mapper(data)
+    out = Fluent::Plugin::RemoteSyslogOutput::SeverityMapper.map(data[:in])
+    assert_equal(data[:out], out)
+  end
 end