A desired signal is received at first and second antennas (111, 112). Output signals of the first and second antennas (111, 112) are orthogonal-detected by first and second signal conversion circuits (121, 122, 131, 132) and thereby converted into first and second baseband signals. A complex conjugation circuit (14) generates a complex conjugation signal by calculating a complex conjugation of the second baseband signal. The first baseband signal and the complex conjugation signal are multiplied by a multiplier (15). A direction estimation circuit (16) estimates a direction of the desired signal by effecting an inverse tangent calculation and an inverse cosine calculation on an output signal of the multiplier (15).