A mix door D, which is arranged in a limited space defined between an upstream air passage 10 in which an evaporator 3 is installed and a downstream air passage 11 in which a heater core 4 is installed, is of a sliding type. By guiding a door proper 12 by using a cam groove, a seal member 15 bonded to the door proper 12 is pressed against a contacting member 13 only when the door proper 12 assumes its close position. Smoothed operation of the door proper 12 is achieved with a compact construction of a unit, and sealing and temperature controlling performance is increased.