The present invention relates to an air conditioner for a vehicle. The air conditioner for a vehicle according to the present invention includes: a plate member slidably operating inside an air-conditioning case to control the degree of opening of air outflow ports or air passageways inside the air-conditioning case; a gear shaft for operating the plate member, and separation preventing means formed on rail portions of the plate member interlocking with the gear shaft.According to the present invention, the gear portions of the gear shaft can be smoothly interlocked with the gear holes formed on the rail portions of the plate member and it is prevented that the gear portions of the gear shaft in interlock with the gear holes of the rail portions are separated from the gear holes or mislocated.