PURPOSE: To enhance the durability, reduce NOx, and lessen the rate of fuel consumption of an internal combustion engine having a mechanism to inject fuel and water into the combustion chamber by performing water injection into the chamber in the first half of the compression stroke.

CONSTITUTION: While a piston 1 is rising near the lower dead point in the compression stroke, a water injection device 5 sends water through a water injection pipe, and the water is injected into a cylinder 7 from a water injection valve 3 to become water atomization 4. The water atomization 4 evaporates and sinks the internal temp. of the cylinder 7, and then sinks the temp. of the flames generated, so that NOx reduction can be expected in the same degree as according to the conventional technique. Because of low internal pressure of the cylinder 7 at the time of water injection, the durability and reliability of the water injection system can be enhanced to a great extent. Also the rate of fuel consumption can be reduced because the internal air temp. of the cylinder 7 can be lowered.