A signal processing method for a plurality of reception signals, wherein a plurality of reception elements are each configured to receive acoustic waves and output a reception signal at a plurality of sampling times, includes generating a representative value signal from the plurality of reception signals, except for reception signals having amplitudes that are larger than a threshold value among the plurality of reception signals at each sampling time, generating a plurality of differential signals indicating differences between the plurality of reception signals obtained at each sampling time and the representative value signal, and generating image data using the plurality of differential signals.