To provide an outlet insertion plug coming-off prevention instrument that prevents a plug from easily coming-off from an outlet.

An outlet insertion plug coming-off prevention instrument 10 includes: a ring-shaped member that is screwed into a plate installation hole of an outlet box; and a hook-shaped member that is installed to a cable of which a plug is inserted into an outlet, and is configured so that the hook-shaped member can be hooked onto the ring-shaped member. In addition, an outlet insertion plug coming-off prevention instrument 20 includes: a hook-shaped member that is screwed into a plate installation hole of an outlet box; and a ring-shaped member that is installed to a cable of which a plug is inserted into an outlet, and is configured so that the ring-shaped member can be hooked onto the hook-shaped member.