PURPOSE: To facilitate carrying a storage box by providing both ends of an almost left-laid U-shaped handle turnably in an upper end part of the storage box, in a device of providing the storage box removably in a lower part side space part of a cushion main unit provided with a front end part serving as the center.

CONSTITUTION: A seat cushion 11 is provided turnably toward the front by a hinge pin 6, in the front part of a seat supporting frame 2, and a synthetic resin-made storage box 10 is removably arranged in a lower part side space part of the seat cushion 4. Here, a handle 12, formed by molding a bar material into a left-laid U-shape, is formed by integrally providing bent parts in both ends of the handle, and this bent part is inserted through the center in a longitudinal direction in the upper end of right/left side walls of the storage box 10, to mount the handle 12. Front/rear paired lock means 14 to engage an intermediate part of the handle 12 are provided in an upper end of the storage box 10, so as to respectively hold the intermediate part of the handle 12 fixed by a click of a lock means 14.