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

A device for retaining a person' seated position is provided as seats for vehicles, for example. The device has a seat member and an upper body support member. The seat member has a seat surface on which a person is seated. The upper body support member has an upper body support surface that supports an upper body back of the person. The upper body support surface has a pair of support projections (80) for ilia, which are fit to a pair of ilium projections of the person, the ilium projections being projected rearward when the person is seated on the seat surface of the seat member. The seat surface has a pair of accommodating recesses (40) for ischia, which accommodates therein a pair of ischium projected portions of the person, the ischium projected portions being projected downward.