An IG key monitor sends out an anti-theft instruction to a GAIN switching part, in case that an IG key SW (ignition key switch) was turned to an OFF state. The GAIN switching part receives the anti-theft instruction from the IG key monitor, and then, switches detection sensitivity of an acceleration sensor to second detection sensitivity (which is detection sensitivity available for inclination judgment of a vehicle, and for example, is approximately .+-.2 G). An inclination judgment part judges whether or not a vehicle is inclined on the basis of a detection result of the acceleration sensor whose detection sensitivity was switched as described above, and outputs an anti-theft alarm through an alarm part, in case that an output of the acceleration sensor exceeds 0.1 G.