An engine being a two-stroke internal combustion engine and being a reciprocating engine, comprising: a cylinder; a piston accommodated in the cylinder; a combustion chamber facing the piston such that the piston is provided at the bottom of the combustion chamber; a sliding portion configured to perform a stroke motion together with the piston and provided below the combustion chamber; a hydraulic surface (114a1) of the sliding portion facing a side opposite to the combustion chamber so as to face downward; a hydraulic chamber, which the hydraulic surface (114a1) faces, wherein the hydraulic chamber is provided below the combustion chamber; a hydraulic pump connected to the hydraulic chamber; a compression ratio changing mechanism which changes a top dead center position of the piston with the hydraulic pump; an auxiliary hydraulic chamber, which communicates with the hydraulic chamber, and has a volume changeable in accordance with a hydraulic pressure in the hydraulic chamber, wherein the auxiliary hydraulic chamber is provided below the combustion chamber; a large-diameter hole, which accommodates the sliding portion, includes a bottom surface opposed to the hydraulic surface (114a1), and forms the hydraulic chamber between the hydraulic surface (114a1) and the bottom surface; and a small-diameter hole, which is opened in the bottom surface of the large-diameter hole, and the hydraulic pump is connected to the hydraulic chamber via an oil passage that opens to the bottom surface of the large-diameter hole, characterized in that the small-diameter hole receives a partition piston provided so as to be slidable, and has an interior partitioned by the partition piston into the auxiliary hydraulic chamber and an accommodation chamber, and further comprising an elastic member configured to press the partition piston from the accommodation chamber side toward the auxiliary hydraulic chamber side. a cylinder; a piston accommodated in the cylinder; a combustion chamber facing the piston such that the piston is provided at the bottom of the combustion chamber; a sliding portion configured to perform a stroke motion together with the piston and provided below the combustion chamber; a hydraulic surface (114a1) of the sliding portion facing a side opposite to the combustion chamber so as to face downward; a hydraulic chamber, which the hydraulic surface (114a1) faces, wherein the hydraulic chamber is provided below the combustion chamber; a hydraulic pump connected to the hydraulic chamber; a compression ratio changing mechanism which changes a top dead center position of the piston with the hydraulic pump; an auxiliary hydraulic chamber, which communicates with the hydraulic chamber, and has a volume changeable in accordance with a hydraulic pressure in the hydraulic chamber, wherein the auxiliary hydraulic chamber is provided below the combustion chamber; a large-diameter hole, which accommodates the sliding portion, includes a bottom surface opposed to the hydraulic surface (114a1), and forms the hydraulic chamber between the hydraulic surface (114a1) and the bottom surface; and a small-diameter hole, which is opened in the bottom surface of the large-diameter hole, and the hydraulic pump is connected to the hydraulic chamber via an oil passage that opens to the bottom surface of the large-diameter hole, characterized in that the small-diameter hole receives a partition piston provided so as to be slidable, and has an interior partitioned by the partition piston into the auxiliary hydraulic chamber and an accommodation chamber, and further comprising an elastic member configured to press the partition piston from the accommodation chamber side toward the auxiliary hydraulic chamber side.