To prevent the rotation of a shaft of a second link rod without providing a special locking member.

An operation lever 12A of one lever type and a spool of a control valve 16 are mutually and mechanically connected through link rods 14B, 14A. A lever bracket 24 supporting a lower end part of the operation lever 12A so as to swing is extended downward along the front of the link rods 14A, 14B and is fixed on a vehicle body frame side. A front projecting part 52 detouring the front of the first link rod 14A is formed on the second link rod 14B by cursing it, and this front projecting part 52 overhangs forward through a slit 50 formed on the lever bracket 24.