A storing section stores data of a pull down cooling characteristic indicative of a time-varying mode of reduction in a target temperature drop. For example, when this is a linear function line, a target internal temperature drop degree takes a constant value, irrespective of an operating time. An actual temperature drop degree is computed on the basis of the detected internal temperature. The computed value is compared with a target value read from the storing section. When the computed value is less than the target value, a rotational speed of an inverter compressor is increased via an inverter circuit. When the computed value is larger than the target value, the rotational speed of the compressor is decreased. The speed increases and decreases are repeated so that pull down cooling is performed along the linear line.