An air-conditioner is operated under one mode selected from among three modes, i.e., a cooling mode, a heating mode and an air-mixing mode. Conditioned air temperature is controlled by changing a cooling ability in the cooling mode, a heating ability in the heating mode, and a position of an air-mixing door in the air-mixing mode. When the operating mode is switched to the cooling mode or to the heating mode from another mode, the position of the air-mixing door is gradually changed to prevent an abrupt temperature drop or rise until a difference between a target temperature and an actual temperature becomes smaller than a predetermined value.