A seat structure applicable to both a normal posture and a comfortable posture, in which a sufficiently relaxed resting posture is available and an excellent comfort is obtained while seated. A cushion frame is supported by a rear link plate and a front link plate, set such that the rear link plate within its movable range has a displacement angle larger than a displacement angle of the front link plate, and an angle between a side frame portion of a base frame and the rear link plate becomes smaller than an angle between the side frame portion and the front link plate when the plates are rotated and moved to the forward-most positions. Consequently, a seating surface angle of the cushion frame increases as the rear link plate and the front link plate are rotated and moved forward. An inclination angle of a back frame itself does not vary but the seating surface angle of the cushion frame can be varied according to a movement of a person.