PURPOSE: To enable a positive alarm to be attained when dusts or the like are accumulated and a filter is clogged by a method wherein a first temperature for sensing a temperature within a condensor, a second temperature sensor for sensing a surrounding temperature of an evaporator and an alarm lamp are provided.

CONSTITUTION: This system is provided with a first temperature sensor 11 for sensing a temperature of a condenser 5, a second temperature sensor 12 for sensing a surrounding temperature of an evaporator and an alarm lamp 13 to be tuned on when the first temperature sensor 11 indicates a temperature more than a first set temperature and the second temperature sensor 12 is less than a second set temperature. In this case, during a normal cooling load operation in which a water temperature is lower than T2°C, a contact point 12a is closed and at this time when dusts are accumulated and clogged in the filter, a temperature of the condensor 5 is increased more than T1°C, the contact point 11a is closed and the alarm lamp 13 is turned on. With such an arrangement, the alarm lamp 13 enables a positive alarm of clogged state in the filter.