PURPOSE: To detect danger accurately in accordance with road and prevent collision properly by changing over into highway mode or road mode in accordance with running on highway or road and outputting high frequency signal of electric power which is controlled by a variable attenuation means in accordance with each mode ahead of automobile.

CONSTITUTION: In case of running on highway, a manual selector switch 13 is changed over into the highway mode. A highway mode signal S9 is supplied to a control section 12 from a highway mode signal generating section 10, and a control signal C1 which conforms to the signal S9 is output to a variable attenuator 5. The attenuator 5 attenuates a signal S3 in accordance with the signal C1, and an output signal S4 is output from an antenna 6 as radio wave M1. Since the strength of electric field of the radio wave M1 must be strong on highway, the signal S3 is hardly attenuated by the attenuator 5. On the other hand, in the case of a general road, the attenuator 5 attenuates high frequency signal to be output to the other automobiles and/or obstacle more in comparison with the case of highway mode set by the signal C1 from the control section 12.