A compact power unit with a sufficient clearance between a stud bolt and a fixing shaft of a cam chain tensioner pivot mechanism for supporting a cam chain tensioner guide. The power unit including a crankshaft, a stud bolt configured to fix a crankcase and a cylinder body to each other, a cam chain, a cam chain tensioner mechanism including a cam chain tensioner guide, and a cam chain tensioner pivot mechanism including a pivot plate, a fixing shaft, and a swinging shaft, the fixing shaft and the swinging shaft are arranged so as to straddle the axis of the stud bolt as viewed in the axial direction of the crankshaft.