PURPOSE: To rapidly eliminate uncomfortable air conditioning environment in a vehicle and to improve a passenger service by a method wherein the passenger ratio of a vehicle is detected, and when the detecting value attains a set value, operation of a cooler for a vehicle is switched from a low cooling state to an ordinary state.

CONSTITUTION: During low cooling operation, a low cooling vehicle switch 9 is switched to the low cooling side. By means of a command signal through a cooling command line 6, a cooling command relay 7 is actuated to excite a timer 8 for starting a cooler. In this case, an auxiliary relay timer 13 is actuated through an output contact 101 of a temperature regulator 10, and a contactor 3 for a cooler is excited to start a cooler 4. Meanwhile, when the passenger ratio of a vehicle is increase to attain a set value, a pressure detecting switch 13 is actuated to excite an auxiliary relay 14. In which case, at the same time when a normally-closed contact 141 of the auxiliary relay 14 is opened and a contact 101 and the contact 101 is separated, a normally-opening contact 142 is closed and a different output contact 102 is connected. This constitution automatically switches operation of a cooler from low cooling to ordinary cooling.