Aikikai Foundation Hombu Dojo

Visit Hombu Dojo is the best experience to learn Aikido.

A place to go once at least in your life if you practice the beautiful Aikido, there are several floors with different level and special lesson for only woman, coin locker, shower, about shower I couldn't find with warm water, maybe only cold water?
There were many people even in early morning class, had a lot of fun and interesting training, many different nationalities there. The environment is traditional and nice.

Josef is incorrect: it is more like a 5-10 minute walk from the Wakamatsu Kawada Station on the Toei Ōedo Line (not the Chuo Line), which is two stops (arrives 4 mins after departure) from Shinjuku Nishiguchi Station. Shinjuku Nishiguchi Station for the Toei Ōedo Line is inside the main Shinjuku Station building. If you want to walk from Shinjuku Station (and not take the Toei Ōedo Line subway) it takes about 30 minutes.

Easy to get to: only 2 minutes walk from the Wakamatsu Kawada station on the Chuo subway line near Shinjuku.

Change rooms and showers are on the third-floor, and remember to bring a 100 yen coin for the lockers there. Also, make sure your membership is valid, and you have your Aikikai card.