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

A seat device has an upper body support member having an upper body support surface that supports an upper back of a person. The support surface has aid ridges (110) for shoulder blades. The aid ridges are located to be fit to a range of fifth to seventh ribs of the ribs in a vertical direction of the seat member, and are substantially horizontal in a width direction of the seat device. The support surface is located such that a length from a center line of the support surface in a width direction thereof to a center-line-side end of each of the aid ridges is equal to or larger than a length from a backbone center line to a backbone end of the person and equal to or smaller than a length from the backbone center line to a shoulder blade of the person.