To provide an anti-theft system for a vehicle capable of significantly reducing an output of an erroneous alarm.

This anti-theft system for vehicle, which issues an alarm if invasion of abnormality into a vehicle compartment is detected, includes: a first detector for detecting a tire air pressure of each wheel, a second detector for detecting a change in a transmission passage of an electric wave transmitting in the vehicle compartment; and an alarm device for issuing an alarm if a variation in a tire air pressure exceeds a predetermined value and a change in the transmission route is detected. A first detector is composed of a plurality of tire air pressure detectors installed on each wheel. The respective tire air pressure detectors transmit the detected tire air pressure to a prescribed communication device installed in the vehicle compartment by means of radio communication. The second detector includes a measurement device for measuring a received signal strength indicator (RSSI) of the radio communication and determines that the transmission route of an electric wave transmitting in the vehicle compartment has changed if the measured variation in RSSI exceeds the predetermined value.

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