To fit a storage container easily and rapidly by forming the storage container in such a way as to be insertable in a space between a pair of opposed seat holding parts, locking a seat bottom face part in a detachable state to a lower end part of a plate-like connecting part, and locking the front end part of the storage container in a detachable state to the front end part of the connecting part.

A load storage container 22 is inserted between seat rails 61a, 61b, and locking protrusions 22c1, 22c2 protrusively provided at a tip part are inserted through through-holes 64a1, 64b1 formed in locking parts 64a, 64b of a bridge 63. The rear end part of the load storage container 22 is pressed downward, and locking pieces 17a, 17b protrusively provided at a fender are inserted through an engaging hole 22d1. At the same time, protruding parts 22a1, 22b1 formed at both side parts of the load storage container 22 are locked to lateral front pipes 61a, 61b. The load storage container 22 is thereby locked at both front and rear face parts 22c, 22d and at both lateral side parts 22a, 22b.