An internal combustion engine in which at least two valves (20, 30) are openable and closable when a piston reciprocates in a cylinder, comprising: as the valves (20, 30), an exhaust valve and an intake valve, wherein the internal combustion engine performs, during three reciprocations of the piston: a first-stage intake stroke in which intake is performed by opening the intake valve and moving down the piston; a first-stage compression stroke in which compression is performed by moving up the piston in a state where the intake valve and exhaust valve are closed; a first-stage ignition stroke in which first-stage ignition is performed when the piston reaches a top dead center or the vicinity of the top dead center; a first-stage combustion stroke in which combustion is performed by the first-stage ignition, and the piston moves down; a first-stage exhaust stroke in which exhaust is performed by opening the exhaust valve when the piston moves down; a scavenging and intake stroke in which scavenging and intake is performed until the piston moves up again by opening the intake valve in a state where the exhaust valve is open and before the piston reaches a bottom dead center; a second-stage compression stroke in which compression is performed by closing both of the exhaust valve and the intake valve and moving up the piston; a second-stage ignition stroke in which ignition is performed when the piston reaches the top dead center or the vicinity of the top dead center; a second-stage combustion stroke in which combustion is performed by the second-stage ignition and the piston moves down; and a second-stage exhaust stroke in which exhaust is performed by opening the exhaust valve when the piston moves down, and repeatedly performs these operations corresponding to three reciprocations of the piston. as the valves (20, 30), an exhaust valve and an intake valve, wherein the internal combustion engine performs, during three reciprocations of the piston: a first-stage intake stroke in which intake is performed by opening the intake valve and moving down the piston; a first-stage compression stroke in which compression is performed by moving up the piston in a state where the intake valve and exhaust valve are closed; a first-stage ignition stroke in which first-stage ignition is performed when the piston reaches a top dead center or the vicinity of the top dead center; a first-stage combustion stroke in which combustion is performed by the first-stage ignition, and the piston moves down; a first-stage exhaust stroke in which exhaust is performed by opening the exhaust valve when the piston moves down; a scavenging and intake stroke in which scavenging and intake is performed until the piston moves up again by opening the intake valve in a state where the exhaust valve is open and before the piston reaches a bottom dead center; a second-stage compression stroke in which compression is performed by closing both of the exhaust valve and the intake valve and moving up the piston; a second-stage ignition stroke in which ignition is performed when the piston reaches the top dead center or the vicinity of the top dead center; a second-stage combustion stroke in which combustion is performed by the second-stage ignition and the piston moves down; and a second-stage exhaust stroke in which exhaust is performed by opening the exhaust valve when the piston moves down, and repeatedly performs these operations corresponding to three reciprocations of the piston. a first-stage intake stroke in which intake is performed by opening the intake valve and moving down the piston; a first-stage compression stroke in which compression is performed by moving up the piston in a state where the intake valve and exhaust valve are closed; a first-stage ignition stroke in which first-stage ignition is performed when the piston reaches a top dead center or the vicinity of the top dead center; a first-stage combustion stroke in which combustion is performed by the first-stage ignition, and the piston moves down; a first-stage exhaust stroke in which exhaust is performed by opening the exhaust valve when the piston moves down; a scavenging and intake stroke in which scavenging and intake is performed until the piston moves up again by opening the intake valve in a state where the exhaust valve is open and before the piston reaches a bottom dead center; a second-stage compression stroke in which compression is performed by closing both of the exhaust valve and the intake valve and moving up the piston; a second-stage ignition stroke in which ignition is performed when the piston reaches the top dead center or the vicinity of the top dead center; a second-stage combustion stroke in which combustion is performed by the second-stage ignition and the piston moves down; and a second-stage exhaust stroke in which exhaust is performed by opening the exhaust valve when the piston moves down, and repeatedly performs these operations corresponding to three reciprocations of the piston.