A cipher type anti-theft system for cars using an in-car wiring to transmit a cipher mainly includes an encoder/transmitter and at least one decoder cut-off relay. The encoder/transmitter generates a cipher and converts the cipher from digits to analog signals for loading onto the in-car wiring. The at least one decoder cut-off relay may be installed at any point on any power supply circuit needed to start the car. When the at least one decoder cut-off relay receives a correct cipher, it can decode the cipher and make the power supply circuit to allow start of the car. The encoder/transmitter and the decoder cut-off relay can be installed in the car without producing additional (or direct) wiring and are therefore not easily found from outside. The encoder/transmitter may be an independent and portable unit associated with certain in-car equipment, such as a cigarette lighter, and uses the cigarette lighter holder, for example, as an interface to receive and transmit the cipher.