PURPOSE: To increase two transmission steps by arranging a cylindrical shaft through which the power of an engine is transmitted to a transmission shaft in a idling manner, arranging a switching clutch to the cylindrical shaft in an integrally rotatable manner, and providing a means of arranging one gear in an idle-rolling manner or the like.

CONSTITUTION: A switching clutch 4 and two gears 5, 6 which are freely locked to/released from the switching clutch 4 are arranged to a transmission shaft 3. An intermediate gear body 8 having gears 8a, 8b engaged with the gears 5,6 respectively is arranged in an idling manner to a counter shaft 7 which is parallel to the transmission shaft 3. A clutch 14 is arranged rearward of the intermediate gear body 8. In this case, a cylindrical shaft 21 whereto the engine power is transmitted is arranged to the transmission shaft 3 in an idling manner, and a first gear 5 which is integrally rotatable with the idling switching clutch 4 is arranged to the cylindrical shaft 21, while a second gear 6 is arranged to the transmission shaft 3 in an integrally rotatable manner. An engagement part 22 which is engaged with the clutch 14 in a releasable manner is arranged to the intermediate gear body 8.