PURPOSE: To provide an engine with a remarkably high efficiency by fitting both a piston and slidable cover in a cylinder so as to inject highly pressurized water into a space between the piston and the pit cover.

CONSTITUTION: A piston 2 and a slidable cover 3 are fitted in the inside of a cylinder 1 while both of them 2 and 3 are spaced apart, and water filled in a space between them 2 and 3. When the slidable cover 3 is in the highest position 5, it comes in contact with a presser bar 6 which does not permit the cover to go upward beyond the position. Under the condition where the piston 2 is in the highest position 4, a pressure pipe 10 is to be connected with the cylinder (1) wall just above the piston 2, and the pressure pipe 10 is connected with a high pressure water pump by way of a pressure plug 11 allowing pressurized water from the pressure pipe 10 to be injected from an injection nozzle 12 into the cylinder 1. In addition, the cylinder wall 1 is connected with an exhaust pipe 13, to which (13) an exhaust valve 15 is provided.