Abstract: Disclosed is an air cleaner for use in an internal combustion engine. The air cleaner has an air filter element and a secondary air filter element therein. Secondary air is fed into the exhaust system of the engine from the air cleaner via the secondary air filter element and a reed valve for promoting the oxidation of unburned HC and CO components in the exhaust gas. A partition is disposed on the circumferential wall of the casing of the air cleaner and is arranged so as to extend along approximately half of the circumferential wall. A secondary air passage is formed between the partition and the circumferential wall. The secondary air filter element and the secondary air inlet are disposed in one end of the secondary air passage. The other end of the secondary air passage opens into the atmosphere. A plurality of holes are formed on the partition and are equiangularly arranged along the circumferential wall.