PURPOSE: To increase the heat collecting efficiency of a water heater, by effectively utilizing the solar rays received by heat collecting plates and by transferring heat to a hot water tank as soon as possible, even when the weather repeatedly changes from fine to cloudy weather, by constituting the water heater so that the press-fitted parts of heat collecting plates which are respectively divided into a plural number are collected to the evaporating part of a heat pipe.

CONSTITUTION: The condensing part 3'b in the upper part of the evaporating part 3'a of a heat pipe 3' is dipped in water 11 in a hot water tank 2a. A heat collecting plate part 4" is composed of a plurality of heat collecting plates crossing at right angles with the heat pipe 3. Each heat collecting plate is composed of a rising part 4'a being inscribed in a transmitter body 9, a heat collecting surface 4'b to receive solar heat, a heat transfer part 4'c forming a suitable slope, and a contacted surface 4'd being press-fitted to the evaporating part 3'a of a heat pipe 3'. The heat collecting plates are concentrated and contacted with the evaporating part 3'a of a heat pipe 3' being interposed by the contacted part 4'd, so that heat from the solar rays can be always supplied to an operating liquid 12 by 100%. On the other hand, the length of a heat pipe 3' is enough to be about 2/3 shorter than the length of a hea collecting plate part 4' so that an evaporation-condensation cycle can be performed at faster speed.