PURPOSE: To perform a defrosting operation in sequence for each of the lines and restrict a reduction in capacity refrigeration by a method wherein the operation is controlled by a stored microcomputer in such a way as a defrosting operation instruction signal is forcedly outputted in sequence under a desired order.

CONSTITUTION: When a forced defrosting operation instruction switch 20 is manually operated, timer times t1Wt3 are measured by each of the timer time setting means 25W27. Every time the measurement of each of the timer times t1Wt3 is completed, the measurement of other timer times is temporarily stopped by temporary stop means 31W33 and the defrosting operation instruction signal is forcedly outputted from the corresponding deicers 10W12 for a desired period of time, for example, 10min on the basis of the forced defrosting operation instruction signal output means 28W30, thereby the defrosting operation correspondingto each of the coolant circulation systems AWC is carried out in sequence in the order of the first, second and third order for each of the systems. Therefore, a substantial reduction in refrigerating capacity caused by a defrosting operation is restricted, a low cost and small-sized equipment can be attained.