To prevent start of an engine by any other person and eliminate the necessity for carrying a special device.

A theft prevention device 20 has a start control unit 22 connected with a harness 19. A signal pattern corresponding to a switching position and switching order of an ignition switch 10 by switching operation of an ignition switch key 16 is stored in a start condition storage part 26 of the start control unit 22. A start control part 24 of the start control unit 22 monitors signal outputted into the harness 19 by switching operation of the ignition switch key 16 and operates a relay 28 when its signal pattern agrees with the pattern stored in the start condition storage part 26 to start an engine.