To realize an intermittent water injection device by the best timing by a simple structure.

This water injection device 2 is equipped with check valves in its ceiling part and bottom part, and comprises a sealed water chamber 26 comprising a diaphragm 27 whose bottom part is recessed downward of a thin plate member, a reservoir tank 25 arranged on the upper part of this water chamber 26, a water tank 21 being feed water means for replenishing water appropriately to the reservoir tank 25, an intake piping 22, a feed water pump 23 and a feed water piping 24. The water chamber 26 is attached under an airtight condition to the upper part of the cylinder head 12 of an internal combustion engine so that the diaphragm 27 fronts on a combustion chamber.