PURPOSE: To detect the beam rotation number of unaxisymmetrical beam around antenna direction by individually controlling the received signal phase to at least one of two comparators and shifting the 0 point direction of difference signal from the antenna direction using a phase shifter.

CONSTITUTION: The receiving signals of antenna elements 111 to 114 are distributed with distributors 121 to 124. One of the distributed received signals is supplied through phase shifters 131 to 134 to a comparator 15 and the other received signals is supplied through phase shifters 141 to 144 to a comparator 16. The comparator 15 produces and outputs from the received signals a summation signal Σ1. and difference signals ΔAz1 and ΔEl1 crossing perpendicularly to each other. Similarly, the comparator 16 outputs summation signal Σ2 and difference signals ΔAz2 and ΔEl2. In other words, with the phase shifters 131 to 134 and 141 to 144, input signal phases to the comparators 15 and 16 are controlled and the 0 point direction of the difference signals is shifted from the antenna direction to detect the rotation angle around the antenna direction.