A wall panel fastening structure for mounting and demounting each panel without moving the other panels is provided on a supporting frame by using a projecting and retracting pin, wherein the wall panel is provided with a wall board part and a fastening part projecting on a back surface of the wall board part; the supporting frame is provided with an accommodating space for accommodating the projecting and retracting pin, and placed along a gap between adjacent wall panels and facing the fastening part; the accommodating space is open to the gap; the projecting and retracting pin is provided with a thread portion and screwed into a thread hole provided in a flange of the supporting frame; and the wall panel is locked to the supporting frame by inserting a front end portion of the projecting and retracting pin into a locking hole formed in the fastening part.