The inner product of noise eigenvalue vectors and azimuth vectors is transformed using an FFT and a MUSIC spectrum is calculated (Step 15). Alternatively the inner product of signal eigenvalue vectors and azimuth vectors is transformed using the FFT and a MUSIC spectrum is calculated (Step 17). A DOA is then estimated based on the MUSIC spectrum obtained (Step 16), thereby decreasing the quantity of calculations required for detecting a DOA of an incident wave using MUSIC algorithm.