To provide a device which is inexpensive, hardly breaks, certainly prevents looseness, and easily cope with a larger outrigger.

The outrigger O is constituted such that an inner box 2 slidingly engaged in an outer box 1, and a vertical outrigger box 3 is placed at the outer end of the inner box 2. A pin guide 6 is provided for the outer box 1, a pin 5 which is energized to the inner box 2 side by a spring 4 is fixed to the pin guide 6, a pin hole 9 with which the pin 5 is engaged in its accommodating condition is placed in the inner box 2, and a positioning member 10 which abuts to the vertical outrigger box 3 in accommodating condition and regulates motion inward of the inner box 2 is placed on the outer end of the outer box 1.