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 (V) 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 (V) 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. <EOS>