To install a ventilating opening window at the stage of mortar finish by repeatedly using a form by forming an under floor ventilating opening by use of a form, removing the form after concrete is hardened, and then finishing the ventilating opening window in the ventilating opening at the finish stage with mortar.

A metal form and a form 1 are made closely adhere to each other without clearance between the surface of the form 1 and the corresponding continuous footing forming frame plate 6 and between the back of the form 1 and the frame plate 6 and fixed, and then concrete is placed in the continuous footing forming space. After hardening, the form 1 and the frame plate 6 are removed to form an under floor ventilating opening through the continuous footing 11. A ventilating opening window is installed in the under floor ventilating opening 10 in such a manner that at the time of mortar finishing for the continuous footing 11 after the end of works for the continuous footing 11, first the ventilating opening window is fastened in the under floor ventilating opening 10 by nails or the like, and mortar 3 is applied thereto from above to fix the window in the under floor ventilating opening 10.