A power unit having a crankcase and peripheral devices wherein a driving source for a gear shifting device is disposed so as to be displaced below the crankcase. An engine includes a gear speed change mechanism having a plurality of gear pairs for changing the speed of a rotational force of a crankshaft. A gear shifting device includes at least a plate-shaped shift arm rocking about a shift spindle to perform a gear pair changing operation on the gear speed change mechanism. A crankcase houses the gear speed change mechanism and the gear shifting device with an oil pan attached to a lower portion of the crankcase. A communicating hole communicating with the oil pan is formed in a bottom portion of the crankcase with the bottom portion being located below the shift arm, and a rocking range of the shift arm partly overlaps the communicating hole.