A 4-stroke cycle internal combustion engine includes a breather chamber for separating blow-by gas and oil mist produced in the internal combustion engine. A vehicle-carried 4-stroke cycle internal combustion engine includes a valve chamber wherein a valve motion is accommodated between a cylinder head and a head cover, a cam chain chamber formed from the cylinder head and the cylinder head cover for accommodating a cam chain, a driven sprocket wheel around which the cam chain is wrapped, and a breather chamber formed between an upper portion of the head cover and the valve chamber. A barrier is formed in an arcuate shape opposing to and extending along an outer circumference of the driven sprocket wheel. The barrier is provided sidewardly of the driven sprocket wheel on the breather chamber side.