Seiji's apartment is GREAT as described in the ad !
It's VERY clean, large, close to Shinjuku-Sanchome station (5 mins walk). On top of that Seiji is very helpful, communicative which made our stay extremely comfortable (not to mention best tea/coffee and other little things that made us feel at home)
We definitely will look for it again when we visit Tokyo again (hope that will be soon!).
Thanks Seiji, for being such great host and having such great place to share!