To heighten combustion efficiency of a gasoline engine so as to reduce contaminant concentration in exhaust gas of the gasoline engine by providing a water supply device for feeding a water solution, preserved inside, into an EGR device in the gasoline engine provided with the EGR device.

A water supply auxiliary system 10 is provided with a small water box 14 fed with water from a large water box 12, and a feed water pump 32 is started by the output of a small water box sensor 30 for detecting the water level in the small water box 14 when the water level becomes the set value or less, so as to feed water in the large water box 12 to the small water box 14. Water in the small water box 14 is fed to a trifurcated head 22 through a feed water pipe 16 and a control valve 18 and sprinkled into exhaust gas fed to a second end of the trifurcated head 22, and the exhaust gas containing water is led into an engine 100 through an EGR device connected to the third end of the trifurcated head 22, which serves to increase combustion efficiency of the engine. The control valve 18 is opened/closed by controlling a current to a relay 40 by a control unit 50.