Jazz Bar Impro is small, even by the incredibly small standards of most Japanese jazz joints. It's on the first floor of a rickety old building down the Heiwa Kouji Yokocho alleyway next to JR Ooimachi station in Shinagawa Ward. This is the kind of alleyway that used to be found all over Tokyo, full of small drinking dens and izakaya run by the locals, sadly all too few these days. Impro has only been open for eight years but fits right in to the old school vibe of the neighborhood.
The entrance into Impro is a very low door you have to duck into, which immediately puts you next to the bar. There's room for maybe 4 to stand and drink, with a tiny table at the back corner for a tight 3 to fit. The master Tsukahara-san is a friendly guy and will make you feel welcome even if you're alone, though I wouldn't recommend the place if you're even slightly claustrophobic.
Tsukahara-san plays a mix of older and more modern jazz on both vinyl and CD, everything was solid the last time I visited, some old Django Reinhardt followed by Lee Morgan. He's got some small snacks on the menu though nothing too elaborate; you wouldn't come to a place like Impro for the food anyways. Impro won't be for everyone (did I mention it's very very tiny and cramped?) but I love the joint. It's a time warp to Showa-era Japan, where every station area had dozens of standing bars like this. Open from 8pm - midnight most nights.
See photos of Impro here at Tokyo Jazz Joints.com.
Come out the east exit at JR Ooimachi Station and turn left. Walk down five minutes (past LAOX dept store) then turn left into the small alleys packed with places to eat. You'll see Impro on a white sign with other very tiny bars in the same building. The alleyway has a sign that says 平和小路.