Disclosed is an air conditioner for a vehicle which has inside air intake ports formed on the side surface of a cylindrical intake duct and the side surface of a cylindrical intake door so as to partially inhale inside air during an outside air inflow mode, thereby reducing manufacturing processes and manufacturing costs because there is no need to add a separate component for inhaling inside air in the outside air inflow mode, controlling intake amounts of outside air and inside air through a recess part formed on the outer circumferential surface of the intake door, and enhancing cooling and heating performance by partially inhaling inside air during the outside air inflow mode.