To restrict an upper opening of a recessed part from being excessively opened while enhancing hold feeling relative to an occupant by the recessed part provided on a center of a sitting part, to restrict right and left sitting surfaces for clamping the recessed part from being not separately largely moved respectively, and to sufficiently maintain ventilation property.

The recessed part 25 lengthy in a longitudinal direction is provided on a center of the sitting part 20, an upper side is covered with a seat 30, and the seat 30 is connected to right and left skins. Thereby, getting-in attitude is varied and an upper opening part of the recessed part 25 is restricted so as not to be excessively opened by the seat 30, and the right and left sitting surfaces 21 of the recessed part 25 are restricted so as not to be separately largely moved respectively. Further, the seat 30 is made to a structure of ventilation property, thereby, ventilation property of the sitting part is sufficiently maintained.