PURPOSE: To provide a stable control system for heat accumulation even if a waterlevel in a heat-accumulating tank is varied, by a method wherein the completion of the heat accumulation is controlled by a pressure of evaporation or condensation in a compression refrigerating cycle composed of heat source device.

CONSTITUTION: Cooling heat is accumulated by producing ice on the external surfaces of heat-transfer pipes for an evaporator 5 in a heat-accumulating tank 1. While the cooling heat is being accumulated, the ice on the external surfaces thereof is allowed to grow, the capacity of the evaporator 5 for transferring heat is allowed to lower, and the pressure and temperature of evaporation are allowed to drop, which pressure or temperature is detected by a pressure sensor or a thermal sensor 9 provided at the outlet of the evaporator 5 in the heat-accumulating tank 1. When the pressure or temperature reaches a pressure preset for the completion of heat accumulation or a value of a temperature drop therefor, the operation of accumulating the cooling heat is completed.