diff --git a/README.adoc b/README.adoc
index cedc4b9..8f6ba2a 100644
--- a/README.adoc
+++ b/README.adoc
@@ -150,6 +150,13 @@ $ go install github.com/jiro4989/ojosama/cmd/ojosama@latest
 $ ojosama -completions bash | sudo tee /usr/share/bash-completion/completions/ojosama
 ----
 
+==== Zsh
+
+[source,bash]
+----
+$ ojosama -completions zsh | sudo tee /usr/share/zsh/functions/Completion/_ojosama
+----
+
 == 注意事項
 
 === 変換結果について
diff --git a/cmd/ojosama/cli.go b/cmd/ojosama/cli.go
index c87f974..27cfa15 100644
--- a/cmd/ojosama/cli.go
+++ b/cmd/ojosama/cli.go
@@ -16,15 +16,24 @@ type CmdArgs struct {
 	Args        []string
 }
 
+const (
+	helpMsgHelp        = "print help"
+	helpMsgText        = "input text"
+	helpMsgOutFile     = "output file"
+	helpMsgCharCode    = "input text file encoding. default is utf8. (utf8, sjis)"
+	helpMsgVersion     = "print version"
+	helpMsgCompletions = "print completions file. (bash, zsh)"
+)
+
 func ParseArgs() (*CmdArgs, error) {
 	opts := CmdArgs{}
 
 	flag.Usage = flagHelpMessage
-	flag.StringVar(&opts.Text, "t", "", "input text")
-	flag.StringVar(&opts.OutFile, "o", "", "output file")
-	flag.StringVar(&opts.CharCode, "charcode", "utf8", "input text file encoding. (utf8 or sjis)")
-	flag.BoolVar(&opts.Version, "v", false, "print version")
-	flag.StringVar(&opts.Completions, "completions", "", "print completions file. (bash)")
+	flag.StringVar(&opts.Text, "t", "", helpMsgText)
+	flag.StringVar(&opts.OutFile, "o", "", helpMsgOutFile)
+	flag.StringVar(&opts.CharCode, "charcode", "utf8", helpMsgCharCode)
+	flag.BoolVar(&opts.Version, "v", false, helpMsgVersion)
+	flag.StringVar(&opts.Completions, "completions", "", helpMsgCompletions)
 	flag.Parse()
 	opts.Args = flag.Args()
 
diff --git a/cmd/ojosama/completions.go b/cmd/ojosama/completions.go
index e392295..a222370 100644
--- a/cmd/ojosama/completions.go
+++ b/cmd/ojosama/completions.go
@@ -27,7 +27,7 @@ _{{APPNAME}}_module() {
           COMPREPLY=($(compgen -W "${opts}" -- "${cur}"))
           ;;
         -completions)
-          local opts="bash"
+          local opts="bash zsh"
           COMPREPLY=($(compgen -W "${opts}" -- "${cur}"))
           ;;
       esac
@@ -37,12 +37,42 @@ _{{APPNAME}}_module() {
 
 complete -F _{{APPNAME}}_module {{APPNAME}}`, "{{APPNAME}}", appName)
 
+	completionsZsh = strings.ReplaceAll(`#compdef {{APPNAME}}
+
+_{{APPNAME}}() {
+  _arguments \
+    {-h,-help}'[`+helpMsgHelp+`]: :->etc' \
+    -t'[`+helpMsgText+`]: :->etc' \
+    -o'[`+helpMsgOutFile+`]:file:_files' \
+    -charcode'[`+helpMsgCharCode+`]: :->charcode' \
+    -v'[`+helpMsgVersion+`]: :->etc' \
+    -completions'[`+helpMsgCompletions+`]: :->completions'
+
+  case "$state" in
+    charcode)
+      _values 'charcode' utf8 sjis
+      ;;
+    completions)
+      _values 'completions' bash zsh
+      ;;
+    etc)
+      # nothing to do
+      ;;
+  esac
+}
+
+compdef _{{APPNAME}} {{APPNAME}}
+
+# vim: ft=zsh`, "{{APPNAME}}", appName)
+
 	completionsMap = map[string]string{
 		"bash": completionsBash,
+		"zsh":  completionsZsh,
 	}
 )
 
 func isSupportedCompletions(sh string) bool {
+	sh = strings.ToLower(sh)
 	_, ok := completionsMap[sh]
 	return ok
 }