PURPOSE: To simplify the constitution of equipment in the titled generator by performing the formation of heated seawater feeding to an evaporator in a seawater boiler heating the seawater directly with a waste gas.

CONSTITUTION: Taken-in seawater is sent to a seawater boiler 10 through a preheater 51 for heating. The seawater sufficiently heated with the waste gas from the boiler 10 is sent to an evaporator 5 so that a prescribed amt. of the heated seawater may be sent to each block. The seawater in each block is flashed respectively to generate steam, and the generated steam is condensed with the seawater in the preheater 51 to be collected as the fresh water on a tray equipped at the lower part of a condensation chamber 52. The fresh water is taken out to the outside. At the time of the condensation, the seawater in the preheater 51 is preheated with the latent heat of the steam.