PURPOSE: To render removing of frame board unnecessary and extremely improve the workability by providing a jig control mechanism for controlling a jig to form a recessed part on the side surface of concrete in an inward projected state or in a retracted state by passing through a frame body.

CONSTITUTION: An opening 22 is formed on the part of a frame board 21, and one end of a guide cylinder 23 is coaxially attached to the opening on the surface of the outside thereof. A jig 25 is coaxially inserted into the guide cylinder 23. The head part of the jig 25a faces to the side of the opening 22, and a bolt 26 is integrated with the fundamental end part thereof. Furthermore, in the inner peripheral surface of the guide cylinder 23, threads are formed corresponding to the bolt 26, and as the bolt 26 is turned around, the jig 25 can obtain a state projected from and state retracted in the opening 22. In addition, at the end part of the bolt 26, a handle 27 is secured thereto via a rotating board in order to drive the bolt 26. The jig control mechanism is therefore constituted of the guide cylinder 23, bolt 26, handle 27 or the like.