To sufficiently ensure secondary air which is necessary for emission control of exhaust gas with a simple structure by forming a pressure recovery hole circulated to a cylinder chamber in the vicinity of a piston bottom dead point on a cylinder side wall of an engine, and driving a diaphragm pump for supplying secondary air utilizing pressure change in the cylinder chamber.

When a piston descends in an intake stroke, mixture is sucked from an outside. When the piston is led to a bottom dead point, a pressure recovery hole is opened. At this time, pressure in a pressure chamber 416 becomes a same pressure level as that in the cylinder, the pressure chamber 416 is contracted by a spring 407, an intake/exhaust chamber 417 is enlarged, and air is led from an intake pipe 411. When the piston is led to the bottom dead point in a combustion stroke, the pressure recovery hole is opened again, only combustion gas in which gas and liquid are separated in a capacity part flows to a diaphragm pump 40 bypassing a pressure leading pipe 420, and is applied on the pressure chamber 416. The intake/exhaust chamber 417 is compressed by a diaphragm 405, and air delivered from a delivery pipe 410 is supplied to an exhaust pipe as a secondary air for treating exhaust gas.