PURPOSE: To prevent the operation of a circulation pump as a heat collecting circulation pump at night in spite of the absence of the sunshine by a method wherein the starting and stopping control of the circulation pump is cut in the morning and evening by discriminating day and night based on the amount of the sunshine as well as a temperature difference between the outlet of a heat collector and a heat accumulating tank while the starting and stopping control of the circulation pump in a day time is effected by both conditions of the amount of the sunshine and the temperature difference.

CONSTITUTION: The timer contact of a day and night timer TA is put OFF in the early morning and evening, therefore, the circulation pump P is not operated. When it becomes day time, the timer contact of the timer TA is put ON, therefore, the starting and stopping control of the circulation pump P is effected by utilizing both sensors H, M of the amount of sunshine and the temperature difference. When the sunshining time is short, the timer is not operated and the circulation pump is left stopping. On the other hand, when the temperature difference between the outlet of the heat collector 4 and the heat accumulating tank 5 is less than 4°C, the output of the temperature difference sensor M is made OFF and the circulation pump P is stopped. On the contrary, when the temperature difference is existing, the circulation pump P is operated and functions as the heat collecting pump as far as the sunshine is existing while it is stopped when the sunshine is terminating.