A lubrication system for a four-stroke engine configured to lubricate components in a crank chamber and in a valve-operating chamber with oil by supplying the oil by utilizing changes in pressure inside the crank chamber caused by reciprocating movement of a piston while circulating the oil, the oil being stored in an oil reservoir provided separately from the crank chamber, the valve-operating chamber accommodating intake and exhaust valve mechanisms therein and configured to discharge blow-by gas in an oil circulation path from the valve-operating chamber to a combustion chamber, comprising: an oil feeding passage including a suction part, the suction part being positioned so as to stay below a surface of the oil even when the oil reservoir is tilted with the oil stored therein in an amount within a rated range and a position of the surface of the oil is thereby changed, the oil feeding passage is configured such that, when a negative pressure is created in the crank chamber, the oil reservoir and the crank chamber are in communication with each other through the oil feeding passage so that the oil in the oil reservoir is sucked through the suction part and is delivered to the crank chamber, wherein the oil feeding passage has an opening end opened in the crank chamber, the opening end being positioned so as to establish communication with the crank chamber when the piston moves from a position near a top dead center toward the top dead center; a direct passage having a plurality of openings that are spaced apart from each other, are opened in the valve-operating chamber at one ends and are opened in the crank chamber at the other ends, so as to communicate the valve-operating chamber with the crank chamber when a negative pressure is created in the crank chamber, wherein the opening end of the direct passage opened in the crank chamber is positioned so as to establish communication with the crank chamber when the piston moves from a position near the top dead center toward the top dead center; and wherein the oil in the valve-operating chamber is delivered to the crank chamber through the direct passage when a negative pressure is created in the crank chamber; a communication passage that, when a positive pressure is created in the crank chamber, communicates the crank chamber with the oil reservoir to deliver oil mist generated in the crank chamber to the oil reservoir; a supply passage for supplying the oil mist from the oil reservoir to the valve-operating chamber; a valve-driving chamber for accommodating driving components of the valve mechanisms, the valve-driving chamber being disposed in the supply passage; and a return passage for returning the oil in the valve-driving chamber to the crank chamber, the return passage being disposed between a bottom portion of the valve-driving chamber on the oil reservoir side and the direct passage. an oil feeding passage including a suction part, the suction part being positioned so as to stay below a surface of the oil even when the oil reservoir is tilted with the oil stored therein in an amount within a rated range and a position of the surface of the oil is thereby changed, the oil feeding passage is configured such that, when a negative pressure is created in the crank chamber, the oil reservoir and the crank chamber are in communication with each other through the oil feeding passage so that the oil in the oil reservoir is sucked through the suction part and is delivered to the crank chamber, wherein the oil feeding passage has an opening end opened in the crank chamber, the opening end being positioned so as to establish communication with the crank chamber when the piston moves from a position near a top dead center toward the top dead center; a direct passage having a plurality of openings that are spaced apart from each other, are opened in the valve-operating chamber at one ends and are opened in the crank chamber at the other ends, so as to communicate the valve-operating chamber with the crank chamber when a negative pressure is created in the crank chamber, wherein the opening end of the direct passage opened in the crank chamber is positioned so as to establish communication with the crank chamber when the piston moves from a position near the top dead center toward the top dead center; and wherein the oil in the valve-operating chamber is delivered to the crank chamber through the direct passage when a negative pressure is created in the crank chamber; a communication passage that, when a positive pressure is created in the crank chamber, communicates the crank chamber with the oil reservoir to deliver oil mist generated in the crank chamber to the oil reservoir; a supply passage for supplying the oil mist from the oil reservoir to the valve-operating chamber; a valve-driving chamber for accommodating driving components of the valve mechanisms, the valve-driving chamber being disposed in the supply passage; and a return passage for returning the oil in the valve-driving chamber to the crank chamber, the return passage being disposed between a bottom portion of the valve-driving chamber on the oil reservoir side and the direct passage.