An array antenna includes a plurality of antennas capturing a coming radio wave and outputting a received signal respectively. An A-D converter converts the received signal to a digital signal, and a frequency detector detects a frequency of the received signal. A sparse signal processor calculates complex amplitudes, which are coefficients for base vectors, each of the base vectors expressing phases of the antennas of the array antenna receiving a radio wave coming from each direction in determined directions, used in expressing the received signal as a linear sum of a finite number of the base vectors, separates the received signal into direction signals for each direction, and calculates the phase of the each of the direction signals. A signal synthesizer aligns the phases of the direction signals using phase differences calculated from phases of the complex amplitudes, and synthesizes the direction signals.