I came here for dinner one night after work, and I was the only customer. The lady who owns the place was so friendly, we got chatting about vegetarian/vegan food (her English was perfect) and she recommended a tasting plate for me, so that I could try lots of different things. I went with her suggestion, because I have enjoyed this way of eating whilst in Japan. I had miso soup, brown rice, braised tofu, seaweed salad, braised cabbage (I think!) and something that was mashed (and delicious!) It was such a filling and enjoyable meal, and she made me feel so comfortable and welcome, I didn't want to leave. She plied me with yuzu fruit free of charge, because I had mentioned that the heating in the hotel was drying out my skin. And she gave me a business card that she had made, which stated in Japanese characters that I didn't eat meat, fish, bonnito etc. so that I could use it in non-veggie restaurants. Such a kind person, she made my evening so enjoyable. Quite easy to find: Take the Chiyoda line to Akasaka station, exit at gate 7, and at street level, turn right. It's literally a 2 minute walk to this restaurant, it is on the right hand side in the basement of a building. No English signage, but I spotted a sign with photos of veggies and healthy looking things on it, and worked it out from that! Well worth a visit.
Cons: Not open at weekends