Seat device having a function of retaining person's seated position

A seat device capable of retaining a person's seated position is provided as seats for vehicles, for example. The seat device has an upper body support member having an upper body support surface that supports an upper body back of a person seated. The upper body support surface has slant ridges (100) for ribs. The slant ridges can touch the ribs of the person from beneath and are slanted according to the ribs, the ribs descending with person's exhalation.