To provide a transmission shaft support mechanism, which includes a power unit case that contains a lubricating oil supply mechanism, and a gear shaft that supports a group of gears for power transmission. An intermediate shaft holder is contained in the power unit case for supporting an intermediate shaft having an idle gear so as to be attached to an intermediate shaft holder attachment surface formed on an inner wall of the power unit case such that the lubricating performance for the intermediate shaft is improved and the lubricating mechanism is simplified. Bolt holes are formed in the intermediate shaft holder and the intermediate shaft holder attachment surface. The oil supply passage for lubricating the intermediate shaft is formed along the bolt holes.