To stably and smoothly raise and lower inner legs by optimally adjusting the gap and the sliding resistance between the mutual sliding faces of an outer leg and the inner legs.

A guide member 37 including a cylindrical sleeve 37a through which the inner legs 6, 7 are inserted at the upper end portion of the outer leg and a pair of movable sliders 42, 42 mounted on the inner face of the sleeve 37a so as to be able to adjust an inward protruding amount and configured that inner faces in a slidable contact with the outer faces of the inner legs 6, 7 is fixed such that the sleeve 37a is fit into the upper end portion of the outer leg.