PURPOSE: To eliminate the friction loss, by employing a labyrinth piston with which a cylinder does not come in contact and wherein a cooling system is inpedently and separately provided.

CONSTITUTION: To the upper part of a piston rod 6 of the labyrinth piston 3 that has a cooling water partition wall 17 and a cooling water supply inner pipe 18 therein, are provided guides 7, 8 for the rod 6, and the guides 7, 8 are spaced apart to have enough ability to retain the rod 6 centrally. A crank shaft 13 and a connecting rod 12 are arranged adjacent the piston 6, and they have a bent constitution to reduce the overall height. Cooling water is introduced via supply inner pipes of second and first swingable arms 16, 15 into the inner pipe 18, is cycled through the circumferential wall of the piston 3 to cool it, rises through an annular passage outside the inner pipe 18, and is led outside via annular passages in the arms 15, 16.