PURPOSE: To improve reliability in an antitheft circuit as well as to make sure of alarming operation, by using an optical fiber for the whole lot of circuit being spread all over the inside and outside of a car body.

CONSTITUTION: An optical fiber 3 reaches to an optical switch passing through a floor wall surface and inside a wall surface in a car body. The optical switch 4 is attached to a part of the whole of a door 8, a window, a trunk lid, etc. Light running past the optical switch 4 immediately passes through the optical fiber 3, centers a photoelectric transfer element 5 being set up in position in close proximity to the tip, converted into a quantity of electricity, amplified by an amplifier 6 and enters an operating circuit of an alarm 7. In this case, for a while an electric current is switched on, the alarm will not operate at all. When the door 8 is opened, the optical switch is interrupted in its circuit whereby no light passes in the optical fiber being after that, namely, the alarm 7 operates in this case.