To reduce the influence of noise such as engine sound, and collect sounds in necessary directions and improve the judging precision for the circumferential state of a vehicle by providing a selecting means which selects a sound collecting means and a judging means which judges the circumferential state of the vehicle according to the sounds gathered by the selected sound collecting means.

A microphone switching part 5 for sound collection selects microphone pairs 1-4 to be used according to the judged result of a travel state judging part 9 and supplies the output signals of the selected microphone pairs to a vehicle discriminating part 10. The vehicle discriminating part 10 discriminates whether or not another vehicle is present or approaching according to the inputted sound signals and inputs the result to an alarm part 11. The alarm part 11 warns the driver with, for example, a synthesized voice when it is judged that the car is approaching. Thus, the circumferential state is judged according to the sounds collected by the selected microphone pairs according to the travel state of the vehicle, so the influence of noise is reduced and the judging precision of the circumferential state of the vehicle can be improved.