To provide a jig capable of easily adjusting a height of a columnar support-mounting fixture.

A height adjustment jig 1 for a columnar support-mounting fixture is capable of freely adjusting a height of a columnar support clamp device 5 attached to a columnar support rod P. The height adjustment jig 1 includes: a clamp portion 2 attached to the columnar support rod P; a male screw portion 3 provided in the clamp portion 2; and a height adjustment nut 41 which is screwed on the male screw portion 3 from an upper part, wherein a top part 41b of the height adjustment nut 41 abuts on the columnar support clamp device 5. In addition, a descent prevention nut 42 which prevents a descent of the height adjustment nut 41 is screwed on the male screw portion 3, and the top part 41b of the height adjustment nut 41 is formed thin.