To simply confirm whether a detaining force of a screw satisfies a required value or not in a short time when a drop wire mounting fitting is attached to an external wall of a house through the screw.

A screw detaining force measuring device consists of a lever plate 11 having a predetermined length, an oscillating key bar 12 having a screw head fixing fitting 12B capable of fixing a screw head part, a support plate 13 provided with a predetermined length in the longitudinal direction of the lever plate 11, protruding in the orthogonal direction for the lever plate 11, and having a tapered shape, a support pedestal 14 supporting a tip part of the support plate 13 by a pivotal shaft so as to oscillate, a bending measuring bar 15 provided on a side face of the lever plate 11 and provided in a section where its mounting base part is overlapped on the support plate, and a scale bar 16 provided in a tip part of the bending measuring bar 14 at a right angle.