PURPOSE: To contrive to enlarge the light receiving area of a water heater by a structure wherein a hot water tank made in the form of a thin plate is arranged at the back of a collector made in the form of a thin plate.

CONSTITUTION: A heat insulating sheet 8, the thin plate heat accumulator 9 and the thin plate collector 7 are housed within a thin box-like upwardly open outer box 10. Furthermore, a transparent plate 6 is placed on the box 10 so as to cover the entire box 10 and fixed with a transparent plate clamp 5 to the box 10. By means of connecting bends 11, the feed water pipe 12 of the heat accumulator 9 is connected to the downcomer 13 of the collector 7 and the return pipe 15 of the heat accumulator 9 is connected to the hot water supply pipe 14 of the collector 7. Owing to the structure as described above, the hot water arrived at the upper end part of the risers 16 of the collector 7 at heat collecting enters through the connecting bend 11 into the upper part of the heat accumulator 9. The stored hot water, the temperature of which turns to be lower, enters from the heat accumulator 9 through the feed water pipe 12 and the connecting bend 11 into the downcomer 13. Few reverse circulation of hot water within the collector 7 during the time except that of heat collecting occurs because the collector 7 has a same structure as a U-tube.