PURPOSE: To provide a warning state and a warning released state to be switched by integrating a crime prevention key in confrontation with a conventional key switch, in a crime prevention system using a crime prevention receiver which raises the warning when a detection loop is formed.

CONSTITUTION: When a changeover switch SWb of a crime prevention key switch 400 is is switched toward a contact point(b) while a crime prevention key switch 1 is untouched, the loop is formed form a power source terminal D+of a crime prevention receiver 300 to a detecting line ls through a warning state indicating lamp La of a switch 400 and terminals T2 and T4 of the switch 1 and the warning released state is obtained. When changeover switches SW1 and SW2 of the switch 1 are switched toward a contact point (a) while the switch 400 is untouched, the loop is formed from a terminal D+ of the terminal 300 to on a power source line l2 through the warning state indicating lamp La of the switch 400, a warning state indicating lamp L1 of the switch 1 and a current limiting resistance R1 and the warning state is obtained.