PURPOSE: To perform stable air conditioning by preventing outside temperature signals from following environmental temperature when the temperature around an outside temperature detector is increased higher than the true outside temperature.

CONSTITUTION: In an on-vehicle air conditioner 20, the conditioner is provided with a judging means which judges whether the environmental temperature around an outside temperature detector 13 is being increased or decreased based on outside temperature signals from the outside temperature detector 13 wherein the outside temperature signal is gradually increased at a specified rate while a limitation is being imposed when environmental temperature is judged by the judging means to have been increasing, and is also provided with a control means 18 which allows the conditioner to respond to outside temperature so as to be changed correspondingly when environmental temperature is judged to have been decreasing.