I noticed this sign at JR Okachimachi station exit when I went to buy supplements the other day.
I asked what it is a station staff, but he has no idea whatsoever. "I don't know about it at all. When this is set up there? "
But it seemed that there must be something near there, so I started look for in its neighborhood, then...
found it! An under the JR track was cleaned up and get painted with white to become a small town.
2k540 AKI-OKA ARTIZANという名前だそうです。
Their name is "2k540 AKI-OKA ARTIZAN."
The first five digits is read "knee-kei-go-yon-maru," it means that to indicate a place it's always start from Tokyo Station by railway technical terminology. And this place is located around 2km and 540m from there. That's why (from official website).
They will be open at 13:00 on this Friday.