To provide a come-off preventing structure for a unit screw for preventing the screw from coming off a mounting part while idling at a certain position as loosened.

The unit screw has a male screw portion. A mounting bracket has a boss which is connected with part of a plurality of cutoff portions of a receiver supporting plate to be fitted thereinto in different directions so that one fitted cutoff portion of the receiver supporting plate is selectively mounted on a receiver case. It therefore consists of a valley-diameter female screw portion to be threaded to the male screw portion of the unit screw and a storage recessed portion for storing the male screw portion of the unit screw. The first male screw portion reaching the storage recessed portion easily passes through the female screw portion and it cannot be returned to the original position to prevent the come-off. It can be moved to the other connected and fitted cutoff portion of the receiver supporting plate. The male screw portion is threaded and fastened to the valley-diameter female screw portion of the mounting bracket again to fix the unit screw to mounting bracket.