PURPOSE: To avoid a wasteful defrosting instruction by selecting the number of input pulses by the atmospheric temperature or humidity in counting the operation time of a compressor by an electornic counter.

CONSTITUTION: Frequency divider 2 devides the frequency of pulse source 1 by half, and atmospheric condition input terminal detects the atmospheric temperature so that, for example, signal 1 enters it when the atmospheric temperature is 15°C or above and signal 0 enters it when it is below 15°C. When the power source is turned on, first counter 8 is reset, and subsequently, or at the same time, the power source impression signal is input from power source impression signal input terminal 9 to flip-flop 10, and signal 1 is output. By this, corresponding to the operation of the compressor, the pulse is input to counter 8 via AND circuit 4, and when the cumulative operation time of the compressor becomes, for example, 8 hours, defrosting signal 12 is output from counter 8. After the completion of defrosting, the input of counter 8 is subjected to the control of the atmospheric condition input.