To enable to continue cooling by avoiding unnecessary stopping of a compressor in case the rotational speed of an engine drastically decreases by decreasing the output frequency of an inverter when the deceleration value of the change is predetermined value or more when the rotational speed of the engine is decelerating.

A controller 10 counts the output pulse signal output from an AVR 12 according to the rotation of a generator 2, detects the rotational speed of an engine 1 from the counted value and detects the deceleration of the change of the rotational speed based on the change of the counted value when the detected rotational speed of the engine 1 is decelerating. When the detected deceleration value is predetermined value or more, the output frequency of the inverter is decreased to the predetermined value. Therefore, if the rotational speed of the engine drastically drops, the unnecessary stop of a compressor 21 can be avoided and the cooling can be continued, and the reliable refrigerator for the refrigeration car having always steady refrigerating performance can be obtained.