PURPOSE: To facilitate the control of the system in response to the change in sunlight quantity and to efficiently perform the steam generation by sequentially connecting in a ring form a pressure valve, load, a water supply tank, a pressurizing pump and a control valve between a steam outlet and a feed water inlet of an air-water separator, and by opening the control valve below a set water level in the air-water separator and actuating the pressurizing pump.

CONSTITUTION: When the water level in an air-water separator 12 is low, a first control valve 16 is opened and a pump 7 is actuated to supply the water in a tank 6 to the separator 12, and so the separator is always maintained at a set water level. While a circulation pump 13 is continuously operated under such a condition to circulate water between a heat collector 1 and the separator 12, the water is heated in the heat collector 1 to be partially made into steam therein. Although the amount of steam may vary according to a change in the sunlight intensity, the steam generation efficiency is good as the heat collector is used under a fully filled condition. When the feed water to the separator 12 is interrupted, a second control valve 14 is opened to introduce the atmospheric pressure into the separator 12, and at the same time the first control valve 16 is opened by being deelectrified or by a drop in the water level in the separator 12 to allow a feed water into the separator 12 through a feed water pipe 19 due to a water pressure.