Maguro Mart
Founded 2011. Only fresh (never frozen) bluefin tuna from the best seasonal sources. Yahoo! Map 4.31 rating. 1,376 Tabelog reviews.
"A place where you can enjoy quality fresh tuna this affordably — rare not just in Nakano but in all of Tokyo. So popular that walk-ins are nearly impossible." "The medium-fatty tuna melts in your mouth, and at 4,000-5,000 yen for this quality, it's unbelievable."