Disclosed herein is an air conditioner for a vehicle, which includes: an air-conditioning case (120) having an air passageway for connecting an air inflow port (127) and an air outflow port (128) with each other; an evaporator (130) and a heater core (140) mounted on the air passageway; a blower unit (110) disposed at the side of the air inflow port (127) to blow air to the inside of the air-conditioning case (120); and a separator (150) disposed at the upstream side of the evaporator (130) to divide air, which is introduced into an air inflow passageway (227) of the air-conditioning case (120), into the right and the left. The air conditioner further includes an assembly end portion (152) disposed at an end portion of one side of the separator (150) to be inserted and assembled on the air inflow passageway (227) of the air-conditioning case (120) by forced fitting.