The light source unit has holding portions for holding a socket on a mounting surface facing the socket, and the socket has an engagement portion for engagement with holding portions. The engagement portion has engagement claws biased in parallel to the mounting surface movably oppositely to each other and engaged with the holding portions, and a holding mechanism that holds the engagement claws while they are adjacent to each other so as not to be engaged with the holding portions. The engagement claws and the holding portions are engaged with each other when the light source unit is disposed in the mounting position to the socket, and the engagement claws held by the holding mechanism is released. For this reason, it is possible to install the light source unit to the socket with a weak force.