The present invention relates to a solar thermal system which has improved use efficiency, enables heat acquired in a heat-collecting unit to be quickly used for a heating and hot water supply load, and can be operated in a stable manner. A solar thermal system comprises: a solar heat collector that absorbs solar heat and heats a heat medium received therein; a heat storage tank that contains heating water, includes a first heat storage exchanger and a second heat storage exchanger connected to the solar heat collector through an heat storage pipe at the upper and lower portions therein respectively, and includes a diffuser that diffuses heating return water to the inside; a press sensor sensing the pressure in the heat storage pipe and a circulation pump pressing and circulating the heat medium that are connected to the heat storage pipe; a heat medium supplementary water tank that is connected to the heat storage pipe through a pressing pump to supplement the lack of the heat medium; an auxiliary boiler that has a heating water outlet connected to a heating water supply pipe of the heat storage tank through a 3-way valve for controlling the heating water supply, and a heating water return port connected to the diffuser of the heat storage tank through a 3-way valve for controlling the heating water return, a check valve that is connected between the 3-way valve for controlling the heating water supply and the heating water return port of the auxiliary boiler; and a heating load that is connected to the heating water outlet of the auxiliary boiler and the 3-way valve for controlling the heating water return.