A plurality of image signals having different exposure levels is acquired; low-frequency components are extracted from each of the image signals to generate a plurality of low-frequency image signals; and the plurality of low-frequency image signals are combined to generate a combined low-frequency image signal. A first image signal is extracted from the plurality of image signals; and high-frequency components are extracted from the first image signal to generate a high-frequency image signal. Then, a combined image signal is generated by combining the combined low-frequency image signal and the high-frequency image signal.