To prevent a solar heat collector from being broken even if the drainage of the solar heat collector is neglected to freeze it in the cold season, to simplify a structure and lower cost by providing a float with an upper surface opened which is vertically movable in an inner casing.

A float 5 is housed in an inner casing 3. The float 5 is formed in the shape of a box with an upper surface opened. On the float 5, two or more solar heat collecting fins 6 composed of protruding members are erected or provided upright in inclined positions. On the lower surface of the float, two or more radiating fins protruding downward are provided. The solar heat collecting fins 6 are provided in order to increase a heat collecting area for absorbing more solar energy. The radiating fins are provided in order to quickly transfer solar heat absorbed by the float 5 including the solar heat collecting fins 6 to water in a bottom. The float 5 is housed in the inner casing 3 by holding a small space between the side surfaces of the float 5 and the inner peripheral side surfaces of the inner casing 3.