Abstract: A vehicle antitheft system and method which effect re-registration and erasure of codes without using a tool used exclusively for such purposes. By the time T1 seconds has elapsed from the time a master key, in which a code is registered, is inserted into a key cylinder, turning an ignition switch from off to on is repeated five times. When opening and closing of a door is repeated six times and the ignition switch is turned off and the key is removed from the key cylinder in the next T2 seconds, a mode is changed to a master key code registration mode. When a master key whose code is to be registered is inserted into the key cylinder within T3 seconds from the time when a master key in which a code is registered is removed from the key cylinder, and the ignition switch is turned on within T4 seconds, an identification code to be registered is read from a transponder. When T5 seconds has elapsed, the read identification code is registered in the memory as a registered code.