An alarm sound according to the distance of a dangerous object exiting around a vehicle is created. A reference sound which enables a driver to recognize the distance of the dangerous object when the driver hears the alarm sound, comparing with the reference sound, is created. The alarm sound and the reference sound are generated at the same time or a different timing. It is preferable that the sound volume of the alarm sound be greater than that of the reference sound. Accordingly, the driver can be warned precisely of the distance of the dangerous object from the vehicle with the sound.