To prevent a belt from getting out of the driving and driven pulleys by fitting a rotary member constituting a support part of a cutter blade to the fitting part having an axial line parallel to the advancing direction of the vehicle body, so as to be rotatable around the axial line.

A rotary member 21 is fitted to a fitting part 30 so as to be rotatable around the axial line. As a result, the center of the first output pulley 12 actuating as the driving pulley fixed on the first intermediate shaft 6 and the center of the second input pulley 13 actuating as the driven pulley fixed on the second intermediate shaft 7 are positioned on the extended line of the axis of the rotary member 21. Accordingly, even if the rotary member 21 is rotated to incline the cutter blade 16, the center of both pulleys 12, 13 is always positioned on the extended line of the axsis of the rotary member 21. And hence, the second V-belt 18 is only twisted. And since the positional shifting between both pulleys 12, 13 is not brought, the belt 18 is prevented from getting out of both pulleys 12, 13 and the power is favorably transmitted.