A small center distance is provided between a crankshaft and a gearshift shaft disposed in parallel. A power unit is built compactly in a direction of arrangement of the crankshaft and the gearshift shaft for supporting reliably the gearshift shaft on which heavy, first and second gearshift clutches are supported. A transmission includes a main shaft, disposed in parallel with a crankshaft and rotatably supported in a crankcase, and first and second gearshift clutches. The first and second gearshift clutches change a gearshift position in a first gearshift portion and a gearshift position in a second gearshift portion, respectively. The main shaft includes an outer shaft portion that extends from the crankcase and supports the first and second gearshift clutches. The outer shaft portion is supported by a bearing portion disposed in a front cover on a side opposite to the crankcase across the first and second gearshift clutches.