A general-purpose engine comprising: a crank case having a mounting flange in a bottom portion of the crank case; a cylinder block extending obliquely from one side of the crank case; and a cylinder head connected to an end surface of the cylinder block via a gasket, wherein a crank chamber in the crank case includes oil spray generating means for generating oil spray for lubricating engine parts by scattering lubricant oil reserved in the crank chamber, and upper and lower side portions of the cylinder block and the cylinder head are provided respectively with an upper oil passage and a lower oil passage which communicate with the crank chamber and a valve operating chamber in the cylinder head with each other; and wherein: a unidirectional valve for allowing a flow of a fluid from the crank chamber to the valve operating chamber and blocking a reverse flow of the fluid is installed in the upper oil passage; in a case where the lubricant oil reserved in the crank chamber flows into the valve operating chamber via the lower oil passage when the engine inclines such that the obliquely extending cylinder block further slants, positive pressures transmitted from the crank chamber to the valve operating chamber via the unidirectional valve push the oil, which, once it flows into the valve operating chamber, flows back to the crank chamber via the lower oil passage; the gasket interposed between the cylinder block and the cylinder head divides the upper oil passage into an upstream passage on the crank chamber side and a downstream passage on the valve operating chamber side; the upstream passage and the downstream passage communicate with each other through a through-hole in the gasket; and the unidirectional valve is attached to the gasket so as to open and close the through-hole. a crank case having a mounting flange in a bottom portion of the crank case; a cylinder block extending obliquely from one side of the crank case; and a cylinder head connected to an end surface of the cylinder block via a gasket, wherein a crank chamber in the crank case includes oil spray generating means for generating oil spray for lubricating engine parts by scattering lubricant oil reserved in the crank chamber, and upper and lower side portions of the cylinder block and the cylinder head are provided respectively with an upper oil passage and a lower oil passage which communicate with the crank chamber and a valve operating chamber in the cylinder head with each other; and a unidirectional valve for allowing a flow of a fluid from the crank chamber to the valve operating chamber and blocking a reverse flow of the fluid is installed in the upper oil passage; in a case where the lubricant oil reserved in the crank chamber flows into the valve operating chamber via the lower oil passage when the engine inclines such that the obliquely extending cylinder block further slants, positive pressures transmitted from the crank chamber to the valve operating chamber via the unidirectional valve push the oil, which, once it flows into the valve operating chamber, flows back to the crank chamber via the lower oil passage; the gasket interposed between the cylinder block and the cylinder head divides the upper oil passage into an upstream passage on the crank chamber side and a downstream passage on the valve operating chamber side; the upstream passage and the downstream passage communicate with each other through a through-hole in the gasket; and the unidirectional valve is attached to the gasket so as to open and close the through-hole.