A power supply unit mounting structure comprises two stepped lock pins provided at an opening of a power supply unit storage and a seal provided at the opening, wherein the power supply unit comprises a flange part and a lock module provided on the flange part, wherein the lock module comprises a lock knob which is rotatable, a lock plate provided at a tip of a rotation shaft of the lock knob, and a cam mechanism configured to displace the lock plate in an axial direction, wherein the power supply unit is inserted into the power supply unit storage, by rotating the lock knob, the lock plate is rotated, a rotation of the lock plate is restrained, by rotating the lock knob after restraining the rotation of the lock plate, the flange part is displaced to the opening side, the seal is pressed, and the opening is sealed.