Disclosed is an air conditioner for a vehicle, which makes it possible for an upper case and a lower case to be assembled in correct position without minute misassembly between the upper case and the lower case, thereby making operation of a sliding door smooth. The air conditioner, which includes an air-conditioning case having a plurality of cases and a door disposed inside the air-conditioning case to be able to slide, further includes: a shape-matching part where the plurality of cases are matched each other in shape; and a guide part crossing the shape-matching part to guide a sliding action of the door, wherein a cross section of the guide part at the shape-matching part is formed to be eccentric.