PURPOSE: To make a protecting operation taken only by an on-vehicle station in a desired area by a method wherein a transmitter of the on-vehicle station is made to emit electric waves having a frequency according to a positional information and the band of a band-pass filter of a receiver is also changed into a band containing the frequency according to the positional information.

CONSTITUTION: A control circuit 6 receives a positional information 9 of a vehicle and controls the oscillation frequency of a programmable oscillator 5 so that electric waves having frequencies F1, F2 and F4 for positions X1, X2 and X4 respectively are emitted from a transmitter 3. Simultaneously, the control circuit 6 sets the band of a programmable band-pass filter 7 receiving the output of a receiver 4 so that it becomes as (b) when a progress is made in the direction from X1 toward X2 and as (c) when the progress is made in the direction from X2 toward X1 with the frequency of the oscillator 5 limited as the boundary. A protecting operation is made according to the output of the programmable filter 7.