While an inverter compressor 32 is driven in a control range, an actual temperature drop S is calculated based on a detected inside temperature TR at every predetermined sampling time. Further, a target temperature drop Ac for the inside temperature TR is taken out of data on a cooling characteristic Xc. If the actual temperature drop S is smaller than the target temperature drop Ac, the inverter compressor 32 is controlled to increase a speed thereof. If it is larger, the inverter compressor 32 is controlled to decrease the speed. Inside of a cold storage is cooled down along the cooling characteristic Xc through those controls. Especially after the inside temperature TR has decreased from an upper limit temperature TH and reached the target temperature To, the speed of the inverter compressor 32 is controlled such that the target temperature drop remains at substantially zero (cooling characteristic Xc2).