"Shusai Ken" （酒采 堅） in Oimachi （大井町）
If you don't know this small diner, you may hesitate to drop in there or may pass by without noticing it, but once you know it, you will definitely love it and feel like coming back again!
"Shusai Ken" （酒采 堅）, or simply "Ken" （堅）, is a small traditional Japanese diner run by a middle-aged Japanese couple who once were running a Japanese restaurant in Toronto for about 10 years. Its exterior and interior look perfectly Japanese with a small counter (for about 6 people) and a small table (for 4 people at the most), but the English-speaking couple will entertain you with good Japanese hospitality and delicious food & sake, so you will definitely enjoy the comfortable Japanese atmosphere and conversations with them in English at the counter.
The food, cooked by Kiyoshi-san (the husband, a.k.a "TJ") is great! His cuisine is creative; some are authentic Japanese, but others are mixed with Japanese and Western tastes. You order food and drinks over the counter by looking at the hand-written menu board or by asking what they have for the day, and Kenko-san (the wife, a.k.a "KT") will serve you. She is a cheerful person full of hospitality. She will show you a variety of sake cups and ask you which one you'd like to drink with, so you can enjoy your favorite sake with your favorite cup. The menu doesn’t have prices of each food and drink on it, but you can ask them if you care, and enjoy within the total budget of 4,000 yen to 7,000 yen per person, depending on how much and what you order.
The restaurant is located at only a 3 or 4 minute walk from the Oimachi Station （大井町駅） on the JR Keihin-Tohoku Line, but is also a real hideaway that may be difficult even for a local Japanese to locate as it's in the quiet outskirts of the neighborhood, so I recommend you search it in advance on Google Map with the address "6-11-26 Minamishinagawa, Shinagawa-ku, Tokyo," and ask them on the phone how to get there from the East Exit (the northern side) of the Station when you make a reservation or when you feel likely to get lost near the Station. Reservation is strongly recommended as it's small but has become very popular recently.