PURPOSE: To obtain a 2-4 strokes switching engine which performs scavenging through a check valve of a scavenging port during 4-stroke operation, and improves heat efficiency.

CONSTITUTION: An intake valve 4 arranged on an intake port 3 of a cylinder head 1 is held in a blocked condition during 2-stroke operation, and opened/ closed during 4-stroke operation, for performing intake. A check valve 28 provided on a scavenging port on a lower side of a cylinder 10 is opened in an initial period of an exhaust process of the 4-stroke operation. New air is sent into a cylinder for scavenging from a turbocharger 5 driven by an exchaust energy. The space inside the cylinder is forcibly scavenged in an exhaust process of the 4-stroke operation, so that an amount of the residual gas in the cylinder gets extremely small. The new air receives heat from a wall surface at the scavenging time in the 4-stroke operation, and energy is recovered by the turbocharger 5 and the like, and thus the heat efficiency is improved.