PURPOSE: To make it possible to smoothly operate a gear shift lever without interference with a control handle, by providing a gear shift lever such that it is rotated around an operating shaft which is inclined with its top being remote from a prime mover, and therefore it may be longitudinally turned.

CONSTITUTION: A speed change unit 10 is arranged in the topsection of a tilling gear box 3 with an input shaft being journalled to the top section of the gear box 3. A shift fork 14 is slidably coupled to a support shaft 13 which is journalled to the gear box 3 in parallel with the input shaft 11, and an operating shaft 15 is rotatably laid in the top section of the gear box 3 such that it is inclined with its top being remote from a prime mover, and is secured at its lower end with a shift arm 16. Further, a gear shaft lever 22 is rotatably fitted onto the operating shaft 15. On a slope, if a control handle 7 is turned toward the prime mover to lift up the lever 22 overcoming a spring so that an engaging element 23 is removed from a piece 20, and is then released from the operator's hand after the handle 7 is turned toward the opposite side, the engaging element 23 is urged by the spring to be engaged with the piece 20.