¥100 flat, paid into the fare box as you get off. Children of elementary-school age and holders of a disability passbook pay ¥50; children four and under ride free. There is no change machine on board — carry coins.
It is a 28-seat microbus and it does fill up: if you can't get on, you wait for the next one. On weekend and holiday afternoons the Tsutsujiso → Honmura → Miyanoura Port run is regularly full. Groups of 10 or more must arrange ahead with the Naoshima Tourism Association on 087-892-2299 — an extra bus can be laid on.
Buses are timed to meet the boats at Miyanoura and will usually wait for a late ferry — but not if that would delay the next run, so for anything that matters take one bus earlier than the connection that only just fits. From Tsutsujiso, Benesse's own free shuttle (every 30 minutes, more often on busy days) continues south to Benesse House Museum, the Lee Ufan Museum and Chichu — the town bus does not enter the Benesse site.