PURPOSE: To compose a secondary air introducing mechanism at a low cost, by making a circular groove for accommodating a reed valve on the lower internal circumference of a ring which composes a main part of a check valve mechanism and also making vertical air intakes through the ring.

CONSTITUTION: A secondary air introducing ring 1 is fitted to a male screw 3 of an ignition plug 2 and screwed in a cylinder head 10 to serve as a gasket for sealing airtight, being held between the cylinder head 10 and the flange 2a of the ignition plug 2. When pressure in the cylinder is equal to or lower than the atmospheric pressure, a reed valve 8 is in the lowest position in a groove 6 for accommodating the read valve 8 and the cylinder chamber is communicated with the outer air through air intakes 7, 7' as well as a space between screw thread of the ignition plug 2 and that of the cylinder head 10. When the pressure is higher on the contrary, the valve 8 is lifted to block the air paths.