To provide an internal combustion engine wherein a problem of an outflow of fuel from an air vent can be solved.

In an internal combustion engine 50 having a fuel circulating route supplying fuel to a carburetor 54 having an air vent 60 provided in the upward of a fuel tank 56 to return excessively supplied fuel to the fuel tank 56 and having an air vent 60 provided in a return side of fuel in this fuel circulating route, further, the engine has an oil reservoir chamber 58, the air vent 60 communicates with this oil reservoir chamber 58, and an air bleed part is provided in the oil reservoir chamber 58. In this way, fuel leaking out from the air vent 60 through the return side of the fuel circulating route is from the fuel tank 56 stored in the oil reservoir chamber 58.