PURPOSE: To increase permeating flow of liquid through a separated membrane and enhance efficiency of using energy by heat exchanging raw liquid and heat exhaust of internal combustion engine as driving source for a separated membrane device of reverse osmosis membrane type to heat up raw liquid and supplying the same to the separated membrane device.

CONSTITUTION: After heat exchanging raw water coming through a raw water pump 1 and a pretreatment device 2 with off-gas and cooling water from an internal combustion engine 5 respectively at heat exchangers 8a and 8b, the same is supplied to a separated membrane device 3. Fresh water separated from raw water by an osmosis membrane of a separated membrane device 3 is sent to a fresh water tank 6. By said process, because of the high temperature of raw water at the separated membrane 3, fresh water can permeate at a high efficiency.