Disclosed is an air conditioner having a control box assembly, which enhances the serviceability of a circuit board and reduces production costs. In the air conditioner, which has a casing unit forming an external appearance of the air conditioner, a heat exchanger, a blowing device, and a control box assembly provided in the casing, the control box assembly includes a mounting unit mounting a circuit board; a control box housing the circuit board and the mounting unit; and guide units provided in the control box to guide a rectilinear reciprocating motion of the mounting unit to allow the mounting unit to come into and out of the control box and guide a rotating motion of the mounting unit in the case that the mounting unit is located at a designated position.