A DOHC-type internal combustion engine, in which an intake-side camshaft and an exhaust-side camshaft are disposed in parallel to each other with a plug hole interposed therebetween, an ignition plug being to be inserted into the plug hole, a pair of intake valves (51L, 51R) and a pair of exhaust valves (52L, 52R) are disposed around the plug hole, the intake valves (51L, 51R) being arrayed in a direction along an axis of the intake-side camshaft, the exhaust valves (52L, 52R) being arrayed in a direction along an axis of the exhaust-side camshaft, two pairs of rocker arms (59L, 59R; 60L, 60R) are respectively interposed between the intake-side and exhaust-side camshafts (57, 58) and the intake valves (51L, 51R) and the exhaust valves (52L, 52R), the two pairs of rocker arms (59L, 59R; 60L, 60R) respectively corresponding to the intake valves (51L, 51R) and the exhaust valves (52L, 52R) while respectively including support portions (65a, 65b, 65c, 65d) and valve connection portions (64a, 64b, 64c, 64d), the support portions (65a, 65b, 65c, 65d) being rockably supported by a cylinder head, the valve connection portions (64a, 64b, 64c, 64d) being operatively connected respectively with the intake valves (51L, 51R) and the exhaust valves (52L, 52R), wherein the first and second support portions (65a, 65b) of one pair of the rocker arms (59L, 59R) out of the support portions (65a to 65d) are disposed between the intake valves (51L, 51R) and the exhaust valves (52L, 52R) in a direction orthogonal to the axes of the intake-side and exhaust-side camshafts (57, 58) and around the plug hole, the third and fourth support portions (65c, 65d) of an other pair of the rocker arms (60L, 60R) out of the support portions (65a to 65d) are disposed outside a region between the intake valves (51L, 51R) and the exhaust valves (52L, 52R) in the direction orthogonal to the axes of the intake-side and exhaust-side camshafts (57, 58), wherein a distance between the first and second valve connection portions (64a, 64b) of the one pair of rocker arms (59L, 59R) is set shorter than a distance between the first and second support portions (65a, 65b), a distance between the third and fourth valve connection portions (64c, 64d) of the other pair of the rocker arms (60L, 60R) is set longer than a distance between the third and fourth support portions (65c, 65d), wherein the rocker arm, on one side along the axes of the intake-side and exhaust-side camshafts (57, 58) out of the one pair of the rocker arms (59L, 59R), and the rocker arm, on the other side along the axes of the intake-side and exhaust-side camshafts (57, 58) out of the other pair of the rocker arms (60L, 60R), have a same shape and are commonalized, the rocker arm, on the other side along the axes of the intake-side and exhaust-side camshafts (57, 58) out of the one pair of the rocker arms (59L, 59R), and the rocker arm, on one side along the axes of the intake-side and exhaust-side camshafts (57, 58) out of the other pair of the rocker arms (60L, 60R), have a same shape and are commonalized, and wherein out of the two pairs of the rocker arms (59L, 59R; 60L, 60R) each pair having two kinds of shapes, an inside diameter of a support hole, which is formed in the first and fourth support portions (65a, 65d) of one kind of the rocker arms (59L, 60R), and an inside diameter of a support hole, which is formed in the second and third support portions (65b, 65c) of an other kind of the rocker arms (59R, 60L), are set different from each other.