PURPOSE: To obviate troublesome operations such as needless water draining and associated water supplying for the subsequent operation, securely prevent freezing by draining water when there is a possibility of freezing and securely prevent a heat-collecting plate, a piping, a pump and the like from being broken, by providing a means for detecting a lowering tendency of the temperature of a heat- collecting part in a solar heat collector.

CONSTITUTION: A thermometer 7 for measuring the outside air temperature, a thermometer 8 for detecting the temperature of water in a heat-accumulating tank 2, a photo cell 9 for measuring the sunshine quantity, and a thermometer 10 for measuring the temperature of the heat-collecting plate 1A in the collector 1 are provided. The lowering tendency of the temperature of the plate 1A at the time of stopping the operation is detected from the result of detection by the thermometer 10, and when the lowering tendency at a preset temperature of slightly higher than the freezing point is such as to lower the temperature to the freezing point, a drain cock 12 for draining water from the plate 1A and the piping 11 is automatically opened by a controller 13 utilizing a micro-computer.