A power unit for a vehicle includes first and second clutches, that transmit/interrupt rotary power from a primary reduction gear mechanism to first and second main shafts, arranged at respective ends of the first and second main shafts with selectively established multispeed gears provided among a countershaft, and the first and second main shafts. The primary reduction gear mechanism includes: a single driving gear provided on a crankshaft; a first driven gear supported by a first main shaft for relative rotation therewith at a position adjacent to the first clutch and meshes with the driving gear; an intermediate driving gear rotating with the first driven gear; an idle gear that meshes with the intermediate driving gear, and supported by the countershaft for relative rotation therewith; and a second driven gear, that meshes with the idle gear, and supported by the second main shaft for relative rotation therewith.