Frequency analysis of each of a plurality of reception antennas and each of reception signals received by the plurality of reception antennas is performed, a power spectrum is calculated for each of the reception antennas, a standard deviation indicating a degree of conformity in a peak of the power spectrum among the plurality of reception antennas is calculated, the power spectra are corrected with use of the standard deviation, a peak is detected based on the corrected power spectra, and a target object is detected based on the detected peak.