Abstract: A four-cycle engine (1) structured to introduce fresh air into a cylinder (1a) via an intake port (1d) opened/closed by intake valves (IN1, IN2) and suck exhaust gas back into the cylinder (1a) via an exhaust port opened/closed by exhaust valves (EX1, EX2), wherein the exhaust port has a first exhaust port (1p) and a second exhaust port (1e), and the exhaust gas is sucked in back from the first exhaust port (1p) and secondary air is sucked in from the second exhaust port (1e) to form, in the cylinder (1a), a first temperature layer (T1) at a high temperature mainly composed of the exhaust gas and a second temperature layer (T2) at a temperature lower than that of the first temperature layer (T1) mainly composed of the secondary air.