To provide a technique effective for theft prevention of an air compressor.

An anti-theft system for a portable air compressor 101 comprises: an air compressor main body portion integrally constituted of an electric motor, a compressing portion compressing intake gas and discharging the compressed gas by using the electric motor, and a compressed gas storage tank storing the compressed gas discharged from the compressing portion; a monitoring means 121 monitoring incorrect transfer of the air compressor main body and sending an incorrect transfer detection signal when detecting the incorrect transfer of the air compressor main body; and an alarming means 141 notifying that the air compressor 101 is stolen when receiving the incorrect movement detection signal.