An engine breather system is formed from a first breather chamber communicating with a crank chamber formed within a crankcase, a second breather chamber communicating with the crank chamber as well as with an intake system, a communicating passage connecting the first and second breather chambers, and a one-way valve. The valve is provided at an open end on the second breather chamber side of the communicating passage, so as to prevent breather gas from flowing from the second breather chamber toward the first breather chamber. The one-way valve is provided with a pressure relief hole, which prevents oil from leaking into the intake system accompanying continued operation of the engine in an attitude different from its normal attitude.