PURPOSE: To perform supplementary heating which is less heat loss and high efficiency by a method wherein the solar ray porjected on a rising inclined plane of an outer casing is converged as a light flux having high energy density such as a laser light and the like with a light convergent permeable body and a primary converter, also the heat of said light flux is almost dissipated without attenuation inside of a hot-water tank from a secondary converter and a light emitting device through an optical fiber group.

CONSTITUTION: The solar ray permeated through a solar ray permeable body 6 heats a heat collecting part 3, rises the water temperature inside of the heat collecting part 3. Thereby, a specific gravity difference occurs between a water inside of the heat collecting part 3 and a water inside of a hot-water tank 5, the hot-water inside of the heat collecting part 3 moves toward the hot-water tank 5 side, and the water rised in temperature by heating is replaced in order with the water inside of the hot-water tank 5. The solar ray incidented from a light collecting permeable body 10 is focused on several focal points, reaches a primary converter 13 through light collecting openings 12. The solar ray increased its light flux density by said primary converter 13 is advanced as efficiently refracted between individual core and clad of an optical fiber group 14 and introduced into a secondary converter 15, then the solar heat is dissipated from a light emitting device 16 toward the inside upper part of the hot-water tank 5, thus, the water inside of the hot-water tank 5 is heated.