PURPOSE: To improve the operability, by providing an ultrasonic wave transceiver before and after cars, selecting the transceiver with a switching relay switch controlled with an output of a connecting decision device connected to a call discriminator, and switching the transceiver placed at the front and rear part surely and automatically.

CONSTITUTION: Front and rear ultrasonic wave transceivers 1a, 1b are provided at the front and rear parts of a car and they are changed over by a changeover relay switch 5. A transceiver 4 is connected to a moving contact of the switch 5 and the transceivers 1a, 1b and a call discriminator 2 are connected to fixed contacts of the switch 5. A connection decision device 7 is connected to the discriminator 2, and front and rear momentary switches 8a, 8b and a hook switch 8c are connected to the device 7. The device 2 discriminates the level of a call signal from the transceivers 1a, 1b, the connection of the transceivers 1a, 1b is decided at the device 7 through the result of the discrimination and the selection of the switches 8a, 8b, the switch 5 is controlled and the connection is displayed on a display 9.