To enable formation of a plurality of undercuts in a skeleton surface simultaneously with the construction of the concrete skeleton, and improve binding performance between the concrete skeleton and a finishing material, or between concrete bodies.

Projecting members 2, each having a large diameter at its front end and a small diameter at the other end on a mold-body side, are formed on a face of the mold body 2 on the side where the concrete is placed. The projecting members 2 can leave holes 10, each having an expanded space inside and a shape corresponding to the shape of the projection member 2, in a surface layer of the concrete C. The projecting members 2 are temporarily and detachably fitted to the mold body 2 by the intermediary of a fitting means 5.