An internal combustion engine includes a cylinder of which the interior is divided into two in the longitudinal direction. The lower chamber serves as a mechanical driving portion in which the piston is reciprocated. The upper chamber serves as a power portion in which combustion gas is drawn in, compressed, burned and exhausted. A cylindrical portion with a lid which moves into the power portion is continuously provided on top of the piston. A gap is defined between the cylindrical portion and the inner peripheral surface of the power portion of the cylinder. The inner peripheral surface of the power portion is defined by a heat shield tube fitted in the cylinder. A heat insulating tube is provided on the outer periphery of the heat shield tube.