An antitheft device for a power generator electrically connectable to a work machine, comprising an acquisition unit configured to acquire position information from each of a plurality of power generators, a setting unit configured to set a single area including positions of the plurality of power generators as a movable area based on the position information acquired by the acquisition unit, and a notification unit configured to make a predetermined notification if at least one of the plurality of power generators has moved out of the movable area set by the setting unit.