To mount a planking hardly bitten into the ground and easily slipped off even when bitten thereto on the rod end of a cylinder so as to be retreated to a position where it never obstructs traveling in traveling.

This outrigger comprises a mounting part 5 provided on the rod 4 end of a hydraulic cylinder 3, and the mounting part 5 is slidably connected to a planking 8 by fitting a T-shaped sectional rail 9 protrusively provided on the planking to a protruding sectional groove 7 formed on the lower surface of the planking, so that it is switchable to a use position shown by the actual line and a non-use position shown by the chain line which never obstructs traveling. A ball 12 loaded by a spring is fitted and engagingly locked to the dent 10 of the rail 9 in each position, whereby the mounting part 5 is constrained in each position. The cylinder 3 is connected to the mounting part 5 by a coil spring 6 installed to the rod 4 to prevent the planking 8 form rotation so that it is never imprudently protruded sideways.