VEHICLE THEFT PREVENTION SYSTEM

The invention has an object to economically and conveniently enable starting of an engine without ruining an antitheft function even in a case where an engine start key is lost. An antitheft system for a vehicle according to the invention includes an allowance tool 5C, 5D for allowing rewriting or writing of an ID code, and a start key registration function of collating an ID code of the allowance tool 5C, 5D with an ID code recorded in an immobilizer control unit 17 and rendering an ID code of an engine key 5 corresponding to the ID code of the immobilizer control unit 17 when a result of a collation is affirmative, so that the engine key 5 serves as an engine start key 5A, 5B. Thus, in the case where the engine start key is lost, since a new engine start key can be formed, the engine can be started by this engine start key economically and conveniently without replacing the whole antitheft device.