The present invention comprises a cylinder block wherein vertical reciprocation of a piston is transmitted to a connecting rod then 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 is placed on the upper side of the cylinder head and rotates to interlock with the rotation of the crank shaft and is connected with a primary intake and exhaust cam driving the primary intake and exhaust valve. A secondary cam shaft is placed on the upper side of the cylinder head and rotates to interlock with the rotation of the crank shaft, and is connected with a secondary intake and exhaust cam driving 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. A primary intake and exhaust control unit is installed in the primary intake and exhaust manifold and controls selectively one of intake of mixed gas or air and exhaust of combustion gas. A secondary intake and exhaust control unit is installed in the secondary intake and exhaust manifold and controls selectively one of intake of mixed gas or air and exhaust of combustion gas. An exhaust-connecting pipe connects the primary and secondary intake and exhaust manifolds, and forms an exhaust path for combustion exhaust gas. Therefore, intake and exhaust performance of a four-stroke internal-combustion engine is improved.