PURPOSE: To improve the efficiency of a hot water supply system utilizing the solar heat by a construction wherein two heat accumulating tanks having different phase transition temperatures are employed, said tanks accommodating heat exchangers for heat radiation and heat absorption, respectively, while pipings for heat collection are properly connected to pipings for hot water supply.

CONSTITUTION: The titled hot water supply system comprises latent accumulating tanks 2, 3 utilizing phase transitions from solid phase to liquid phase or vice versa and having different phase transition temperatures. The tanks 2, 3 accommodates heat exchangers 4, 5 for heat radiation, and heat exchangers 6, 7 for heat absorption,respectively. Piping for heat collection is connected to the heat exchangers 5, 4 for heat radiation along the direction of flow of heating medium and in the sequence from the heat accumulating tank 2 for high temperature transition to the heat accumulating tank 3, whilst piping for hot water supply is connected to the heat exchangers 6, 7 for heat absorption along the direction of flow of water and in the sequence from the tank 3 to the tanks 2. Stating briefly, because two latent heat accumulating tanks are provided, heat accumulation can be made even at a comparatively low heat collection temperature resulting in higher daily heat collection efficiency than that of the conventional system, and the heat output efficiency can be improved thereby obtaining a more practical system.