PURPOSE: To obtain a favorable seat inexpensively, by making resin layers on the side where one's body will be in contact with thinner, forming, to said resin layers, prescribed projections and resilient parts in the shape of bellows, making positions opposite to said side thicker, and forming air openings in said positions, in case that seat frames are blow molded from a resin.

CONSTITUTION: In case the seat frames 3, 4 comprising resin hollow components are blow molded, the thickness t1 of the resin layers 3a, 4a on the side where one's body will be in contact with is made thinner than the thickness t2 of the resin layers 3b, 4b on the opposite side (preferably the thickness of the former is 1/2 of that of the latter), the resin layers 3b, 4b are formed with the prescribed projections 5a', 5b', 6b', 6a and the resilient parts 5a", 5b", 6b" in the shape of bellows. And the air openings 7 are provided in the resin layers 3b, 4b.