PURPOSE: To make it possible to generate an induction signal which controls included noise in a degree that its induction accuracy may not be deteriorated, and prevent the responding ability of the signal from being deteriorated more than required.

CONSTITUTION: An angle error detector 33 generates an angle error signal which indicates a differential angle between the direction of transmission beams and the arrival direction of reflecting radio waves. An induction signal δ is generated by passing this angle error signal through an induction signal arithmetic operation unit 42 passing through a specified frequency zone. The passing zone of the induction signal arithmetic operation unit 42 is changed by a filter control arithmetic operation unit 43, responding to the intensity of reflective radio waves which an antenna 1 receives.