Aomori (青森), the northernmost prefecture of Honshu Island and Tohoku Region carries many good onsens. My choice for this post is Kappa-no-yu (かっぱの湯) at Oku-yagen Onsen (奥薬研温泉) in beautiful forest of Shimokita Peninsula.

It's quite simple. Just park your car, walk down stairs toward Ohata River and you'll see the big open-air bath with changing space, which is divided into two parts. A sign board on the wall says "Don't bring in anything but towels" in Japanese. It means swim suites bathing isn't allowed here.

The Kappa is one of the legendary monster spirits of Japan. A Kappa is believed to have carried an injured monk here and cured his injury with this onsen in 9th century. That's why there is a small statue of Kappa by the rim of the bath. I saw someone had left offerings of coins and a glass of sake (Japanese rice wine) to the Kappa.
Hot spring water looks green on the photo, but it's transparent, rather hot at 42-43 degrees when I visited in mid summer. The best season to visit is late October. You'd be surrounded with colorful forest of red and yellow leaves.

◆Kappa-no-yu, Oku-yagen Onsen(奥薬研温泉かっぱの湯)
Type: Gender mixed open-air bath. Swim suites bathing and use of soap & shampoo not allowed.
Address: Oku-yagen, Ohata-machi, Mutsu-city, Aomori Pref. (青森県むつ市大畑町奥薬研)
Available hour: Anytime year round (no lighting facilities)
Holiday: None (but Wed. morning is unavailable for cleaning)
Admission fee: Free
Equipment: None except changing space with shelves. There is a camp site nearby.
Access: It's difficult to visit here by public transportation. From Shimokita railway station of Ohminato-line, take a bus to Ohata Bus Center (45 minutes), then take a taxi (15 minutes). If you are going to stay at a ryokan of Yagen Onsen, you can expect discount rate for taxi or your ryokan might offer free shuttle service between Ohata and Yagen Onsen. Oku-yagen Onsen is about 2km away from Yagen Onsen (Oku-yagen means deep yagen).
My rating (out of five): 4
External links:
Map of Kappa no Yu in English (scale changeable)
Slide show of Kappa no Yu with music (click the green "PLAY" bottun)
You can see more photos here (written in Japanese)
[Note] I have difficulty finding a detailed free on-line map of Japan in English. Google map is written in Japanese, and the map of Yahoo.com is useless, too. If you have any suggestions, please leave me a comment.