A plurality of oil passages are formed in a shaft body of a rocker shaft, which partially constitutes a valve drive system of an internal combustion engine. A bolt hole is formed in the shaft body in such a manner as to substantially perpendicularly intersect the axis of the shaft body. A bush is fitted into the bolt hole in such a manner as to prevent the bolt hole from establishing communication between the plurality of oil passages. A bolt is fastened through the bush to thereby fix the rocker shaft on a cylinder head. This structure improves the support rigidity of the rocker shaft and the dynamic characteristics of the valve drive system, and reduces the number of components and the number of working man-hours.