The edge of an electrical junction box is inserted an adjustable amount between two plates movable toward and away from each other by suitable means, as tongs. This adjusted amount is the amount allowed for the thickness of a wall to be later constructed and to the surface of which, the edge of the electrical box should project. An adjustable rod determines the selected elevation of the box. While the box is being secured to a stud, the jig permits the box to be manually held in the selected position for installation.