The invention comprises a cylinder block wherein vertical reciprocation of a piston is transmitted to a connecting rod that rotates a crank shaft to generate torque. A cylinder head is joined to the top of the cylinder block, whereof one side is connected to a primary intake and exhaust manifold which is opened and shut by a primary intake and exhaust valve and the other side is connected to a secondary intake and exhaust manifold which is opened and shut by a secondary intake and exhaust valve. A primary cam shaft drives the primary intake and exhaust valve. A secondary cam shaft drives the secondary intake and exhaust valve. A tertiary cam shaft is placed on the upper side of the cylinder head and rotates to interlock with the rotation of the secondary cam shaft. Therefore, intake and exhaust performance of a four-stroke internal combustion engine is improved.