PURPOSE: To attain the estimation of a deflection angle of a beam due to a failure by monitoring a signal as a result of multiplication of a specific weight function corresponding to a radiation beam bias charactersitic to a failure detecting signal of a phase shifter corresponding to each radiating element of an array antenna.

CONSTITUTION: A signal inputted from a terminal 51 is distributed to phase shifters 2-(-N)∼2-(N) at a power distributor 3, the phase is shifted according to the radiation beam deflection angle for a prescribed amount and the signal is irradiated at radiating elements 1-(-N)∼1-(N). Each phase shifter has a failure detection output terminal, which is connected respectively to failure signal weighting circuits 5-(-N)∼5-(N), where a weighting function corresponding to the arranging position of a corresponding radiating element is multiplied and the result is inputted to a failure monitor circuit 6. A faulty phase shifter is detected by obtaining a failure detecting signal from the phase shifter without measuring an electromagnetic wave and its radiating beam characteristic is estimated.