Gear Transmission

A detent mechanism is provided for fixing a shift drum in position at a rotation operation position where shift gears become engageable. The detent mechanism includes a positioning rotation portion formed in the shift drum , a positioning arm engageable with a positioning action portion formed in the positioning rotation portion for fixing the shift drum at a rotation position corresponding to a rotation operation position of the shift drum , and a positioning spring for urging the positioning arm for its engagement with the positioning action portion . The positioning arm , as being rotatably supported to a speed changing operation shaft for rotating the shift drum , is extended from a speed changing operation shaft