diff -ur v1/idt.py v2/idt.py
--- v1/idt.py	2018-09-25 22:19:24.000000000 +0900
+++ v2/idt.py	2018-10-02 22:53:13.000000000 +0900
@@ -52,13 +52,11 @@
 
 if __name__ == "__main__":
 	b = nkf.get_stdin()
-	opt = nkf.guess(b)
-	u8 = nkf.cvt(b, '-u')
-	s = nkf.dec(u8)
+	(s, opt) = nkf.to_str(b)
 
 	s = '\n'.join( map( cv, s.split('\n') ) )
 	
-	u8 = nkf.enc(s)
-	b = nkf.cvt(u8, opt)
+	o = get_opt('-o', '')
+	b = nkf.str_to( s, '-' + o if o else opt )
 	nkf.put_stdout(b)
 # EOF
diff -ur v1/nkf.py v2/nkf.py
--- v1/nkf.py	2018-09-18 22:20:14.000000000 +0900
+++ v2/nkf.py	2018-10-02 23:07:15.000000000 +0900
@@ -22,23 +22,45 @@
 	fo = sys.stdout if six.PY2 else sys.stdout.buffer
 	fo.write(b)
 
-def guess(b):
+opt_dic = {
+	'ISO-2022-JP': '-j',
+	'EUC-JP': '-e',
+	'Shift_JIS': '-s',
+	'UTF-8': '-u',
+	'ASCII': '',
+}
+
+def guess(b, style_opt=False):
 	b = do_cmd('nkf -g', b)
-	return dec(b).strip()
+	opt = dec(b).strip()
+	return opt_dic.get(opt, '-u') if style_opt else opt
 
-def cvt(b, opt):
-	if opt == 'ASCII':
+def cvt(b, opt, do_guess=False):
+	if opt not in opt_dic.values():
+		opt = opt_dic.get(opt, '-u')
+	if opt == '':
 		return b
-	d = {
-		'ISO-2022-JP': '-j',
-		'EUC-JP': '-e',
-		'Shift_JIS': '-s',
-		'UTF-8': '-u',
-	}
-	if opt not in d.values():
-		opt = d.get(opt, '-u')
+	if do_guess:
+		from_opt = guess(b, True)
+		if from_opt == '' or from_opt == opt:
+			return b
 	return do_cmd('nkf ' + opt, b)
 
+def to_utf8(b):
+	opt = guess(b, True)
+	u8 = b if opt in ('-u', '') else cvt(b, '-u')
+	return (u8, opt)
+
+def utf8_to(u8, opt):
+	return u8 if opt in ('-u', '') else cvt(u8, opt)
+
+def to_str(b):
+	(u8, opt) = to_utf8(b)
+	return ( dec(u8), opt )
+
+def str_to(s, opt):
+	return utf8_to( enc(s), opt )
+
 if __name__ == "__main__":
 	b = get_stdin()
 	opt = guess(b)