To provide a water heater utilizing solar heat capable of surely preventing freezing.

This water heater comprises a thermal storage tank 10 for storing a thermal storage medium, a heat collector 20 for heating the thermal storage medium by collected solar heat, a water supply circuit 30 directly connected to the thermal storage tank 10 and a water main pipe P, a circulating circuit 40 for circulating the thermal storage medium between the thermal storage tank 10 and the heat collector 20, and a tapping circuit 50 connected to an upper part of the thermal storage tank 10 and the water supply circuit 30. The tap water introduced into the thermal storage tank 10 by water pressure of the water main pipe P is circulated and supplied to the heat collector 20 as the thermal storage medium to produce the hot water, and the hot water is stored in the thermal storage tank 10. When the tap water introduced as the thermal storage medium is likely to freeze, a high-temperature operation solenoid valve 47 is forcibly opened, and the tap water hardly freezing, supplied from the water main pipe P through the water supply circuit 30 is discharged from an escape pipe 46 through the heat collector 20 to prevent the freezing.