PURPOSE: To enhance heat efficiency, by heating a medium by solar energy to exchange heat with seawater and evaporating heated seawater in a distillator while further evaporating seawater in the next stage distillator under the heat exchange of seawater obtained in the previous stage distillator with steam.

CONSTITUTION: The titled apparatus is constituted of a heat collector 1 containing heat collecting pipes 1' for heating a heating medium such as oil, a heat exchanger 2 for exchanging heat of the heated medium and seawater and distillators 3W8 connected to the downstream side of the heat exchanger 2 and obtaining steam through the evaporation of seawater. In addition, heat exchangers 9, 11, 13, 15, 17 for further condensing steam from the distillators 3W 8 to obtain water, gas-water separators 10, 12, 14, 16 connected to the downstream sides of said heat exchangers, a gas extracting ejector 19 for reducing pressure in the distillators, a fresh water pool 20, a pump P1 for recirculating the heating medium to the heat collecting circuit, a pump P2 for pumping up seawater and a pump P3 for sending fresh water of the fresh water pool 20 to the gas extracting ejector 19 under pressure are provided.