An air cleaner is used in a two-stroke internal combustion engine to supply an air-fuel mixture passage and a fresh air passage in a carburetor with air cleaned by an air cleaner element. The air cleaner includes an air cleaner base having an air-fuel mixture opening communicating with the air-fuel mixture passage and a fresh air opening communicating with the fresh air passage, and an air guide member provided at the air-fuel mixture opening. The air guide member is configured to curve near the air-fuel mixture opening and extend from there along the air cleaner base, and has a cross section surrounded by a peripheral wall. The air guide member defines a mixture-use air extension passage for air to be added to prepare an air-fuel mixture air-fuel mixture, which extends from a first clean air intake to the air-fuel mixture opening. Blown-back gas from the air-fuel mixture passage hits against the inner wall of the curved portion of the air guide member, and is therefore restricted to flow out of the first clean air intake and leak into the air cleaner. Thereby, harmful emissions in the exhaust gas from the two-stroke internal engine can be reduced.