To provide an anti-theft device which can prevent malfunctions as much as possible.

The anti-theft device has a doppler sensor part 11 which detects approach/separation of a person to/from a vehicle in a prescribed monitoring area around a vehicle, and a plurality of modes are shifted step by step as follows according to a change of a signal level based on the output of the doppler sensor section 11. In a normal operation, a normal monitoring mode is set for continuously detecting approach/separation of a person in the monitoring area around a vehicle by the doppler sensor part 11. When an abnormality occurs in a signal level in the monitoring mode, it is shifted to a security mode. When an abnormality occurs in the signal level in the alarm mode in a prescribed time, it is shifted to a warning mode, and a suspicious person is warned by sound and light, and when there is no abnormality in the signal level in the prescribed time, it is shifted to the normal monitoring mode.