PURPOSE: To reduce the cost by using in common the common parts of an on-vehicle apparatus such as an audio with theft preventing function, and a security/keyless entry device of a vehicle.

CONSTITUTION: To the front side of a car audio main body 1, a nose piece 3 for audio, and a nose piece 4 for security/keyless entry are formed allowable to convert alternatively and to mount and demount. When the nose piece 3 for audio is installed, an audio device such as a cassette and a tuner is made possible to control. When a driver leaves from the vehicle, the nose piece is converted to the nose piece for security/keyless entry, and as a result, a sensor detects an abnormal force applied to the vehicle, and a security/keyless entry unit 2 connected to the main body 1 drives a means such as a siren. A receiver receives a transmission signal from a remote controller for keyless entry, and the security/keyless entry unit 2 drives a door lock means so as to carry out a locking or an unlocking.