Provided is an internal combustion engine, in which an inlet and an outlet to the combustion chamber are on both sides each formed with intake valve or exhaust valve of a center line of a cylinder head, an intake and an exhaust rocker arm for opening and closing of the valves in each case to the inlet or exhaust valve are arranged, and an intake camshaft and an exhaust camshaft having cams for pivoting the rocker arm are each arranged above the rocker arm. Swivel side end portions of the rocker arms pivot about supporting point side end portions of the rocker arms by a transmitted by the cam force for opening and closing the valves .; The support point side end portion of the intake rocker arm is arranged on an opposite side to the exhaust valve with respect to the intake valve, and the support point side end portion of the outlet rocker arm is disposed between the intake valve and the exhaust valve. A camshaft-side cover surface with respective camshafts lines is parallel to a combustion chamber top surface side orthogonal to the center line of the cylinder head, when the cylinder head by respective camshafts-line directions of the intake and exhaust camshaft is considered. The inlet is disposed below the intake rocker arm. Inlet and outlet valves are arranged so that an axis line of the valves is inclined relative to the cylinder center line by predetermined angles.