PURPOSE: To collect the solar rays effectively without being affected by the variation of a height of the sun, by a method wherein side heat collectors each of interiors of which is provided with a circulation duct of water, are arranged in a facing state and a reflector is provided so that the solar rays can be concentrated toward a central heat collector.

CONSTITUTION: A heat collector 14 is constituted with a central heat collector 16 extending in a short-lengthward direction at the center of the inside of a casing 10 and a pair of side heat collectors 20 and 20 extending in a longitudinal direction respectively in the inside of each of longitudinal side plates 18 and 18 of the casing 10. A reflector 17 is constituted by arranging a large number of long sideways rectangular plane mirrors in order and in parallel within the casing 10 as shown in a sketch. The solar ray, therefore, is reflected by the reflector 17, concentrated and radiated to the central heat collector 16, through which water in a jacket is heated and the heated water is exchanged with water in a hot water tank 32 through natural circulation. Reflected light is radiated toward the inside of a pair of side heat collectors 20 and 20 and the one side heat collector 20, to begin with, and heat collecting efficiency of the solar rays is improved drastically.