To provide an extensible leg locking device in need of only a small number of parts and few assembly man-hours without getting in the way.

Each end part of a horizontal locking member support shaft 12 passing through a locking member 13 passes through a side wall of a rung 3A so as to be supported by the side wall. An elastic member 14 is formed as a torsion coil spring and wound onto the locking member support shaft 12. A lower wall of the rung 3A is cut out to allow the rotation of the locking member 13.