A power unit for a small-sized vehicle that includes a valve operating system equipped with a hydraulic valve operation mode change mechanism capable of changing the operational mode of an engine valve, and a clutch device disposed between a crankshaft and a shift mechanism so as to switch hydraulically between connection and disconnection of power transmission, wherein a discharge outlet of one oil pump (209) driven by power transmission from the crankshaft is connected in common to a valve operation mode change mechanisms (63, 64) and a clutch device (102). This enables the hydraulic system to be made compact while avoiding any increase in the overall bulk of the power unit, thus providing a power unit suitable for a small-sized vehicle.