Whether or not a flow rate of a circulating refrigerant is higher than a predetermined threshold value is judged and when the flow rate is higher than the predetermined threshold value, the discharge flow rate of a compressor 11 is controlled in accordance with the degree of cooling of a front seat evaporator 9 and when the flow rate is lower than the predetermined threshold value, the discharge flow rate of the compressor 11 is controlled so as to intermittently change between a discharge flow rate near the predetermined threshold value and a flow rate of substantially zero and the operation time of the compressor 11 during the period of that control is measured and when the compressor operation time exceeds a predetermined time, the compressor is continuously controlled in accordance with a discharge flow rate near the predetermined threshold value.