How about Taiyaki, a Japanese sweet? It's a fish-shaped Japanese pancake with red bean paste fillings.
There is a place called "Naniwaya Souhonten", which is said that first Taiyaki in Japan was created here. "Naniwaya Souhonten" has more than 100 years of history, many people come here from all over Japan to taste the authentic Taiyaki here.
Here is the link to yelp.
A reasonable Roppongi kaiseki restaurant would be,
I don't know this will be what you think as a "Japanese food" but how about "Teishoku", set meal? You can enjoy homemade style Japanese dishes in very affordable price.