PURPOSE: To automatically set the value of a threshold level that becomes reference for vehicle sensing corresponding to the environmental condition of weather and a road, etc.

CONSTITUTION: The application of the power source of this device is detected at a power source application detecting part 1, and a time counter 10 is reset. Thence, a noise level is set at an initial preset value at a noise level initial preset part 2. The value is stored in noise level memory 4. Thence, the level of a reception signal is measured at every time T1 at a reception level detecting part 6. The level of the reception signal is compared with the noise level stored in the noise level memory 4 by a comparator 7. When it is judged that the level of the reception signal is less than the noise level, the noise level stored in the noise level memory 4 is decreased by a noise level change part 8. When prescribed time T2 elapses after the power source is applied, a value in which a value decided in advance is added on the noise level stored in the noise level memory 4 is set at a threshold decision part 9 as the threshold level.