A bumper device can reduce a weight of a reinforcement member more than a weight of a conventional reinforcement member while sufficiently absorbing a given impact energy. A size “b” between both side mounting portions where the reinforcement member is mounted on a stay is set greater than a size “a” between both side longitudinal wall portions of a side frame, both outer longitudinal walls of the stay are inclined such that both outer longitudinal walls are opened toward the reinforcement member, and the distance between the inner longitudinal walls is set substantially equal to the distance between the both side longitudinal wall portions of the side frame.