Driving power is transmitted to an output shaft (23), via driving and driven bevel gears (21, 22), to a cutter blade (16). A gearbox (24) supports the output shaft via first and second bearings (27, 28). The second bearing (28) is engaged by an engaging member (29) to be prevented from axial movement, toward the blade, relative to the gearbox and supports the output shaft near the cutter blade. The output shaft (23) is connected to the driven bevel gear (22) such that it is prevented from rotation relative to the driven bevel gear but allowed to move axially. When the engaged state, by the engaging member (29), of the second bearing is canceled, the shaft (23) is allowed to axially move toward the cutter blade until the shaft becomes rotatable relative to the driven bevel gear (22), at the same time of which a retaining stopper (41 41A 41) prevents the shaft from further axial movement.
