To realize an intermittent water injection device by optimal timing in simple constitution.

In this device, an upper part is communicated with a water reservoir 21, a lower part cools the inside of a combustion chamber by elevating a piston 24 for water injection by pressure of the combustion chamber by inserting the piston 24 for water injection in a water chamber 23 to open to the combustion chamber, the upper part is communicated with an oil reservoir 31, the lower part inserts a piston 34 for oil circulation in an oil chamber 33 to open to the combustion chamber, and the water injection is efficiently continued by elevating the piston 34 for oil injection by pressure of the combustion chamber to circulate oil, preventing intrusion of combustion gas to each of sliding surfaces including the piston 24 for water injection, and by lubricating and cooling each of the sliding surfaces.