A lock mechanism includes: a lock arm provided in a cantilever manner on an inner wall of a part compartment of a box body of an electric connection box; and a lock projection provided to the lock arm on its surface facing a part housed in the part compartment. The lock arm can be divided in a longitudinal direction into: a base end region fixed to the inner wall; upper end region located in the free end which interferes with the part and receives a pressing force for unlocking operation; and a middle region located within a predetermined distance range between the base and upper end region. The flexural rigidity of the base and upper end region is set larger than that of the middle region by providing ribs on the base and upper end region and providing no ribs on the middle region.