Asahi-yu, somewhere in Shin-Nakano, on the Marunouchi line.
I can't believe this sento is only a few minutes west of Shinjuku. It looks like the ones we used to go to in the mountains 20 years ago. Though I don't remember there being a baby scale with as sweet a patina as this one has. Because nothing polishes quite like the freshly bathed butts of 60 years worth of Japanese babies.