An ROI in a first image and a second image are synthesized by synthesizing first wavelet coefficient data corresponding to the first image and second wavelet coefficient data corresponding to the second image. An ROI coefficient and a non-ROI coefficient in the first wavelet coefficient data are determined on the basis of a mask for the first wavelet coefficient data, and the ROI coefficient and a wavelet coefficient in the second wavelet coefficient data, which is corresponding to the non-ROI coefficient, are synthesized. When the above coefficient synthesis is performed after a decomposition level of the first wavelet coefficient data is increased, the first wavelet coefficient data is adjusted on the basis of auxiliary wavelet coefficient data corresponding to an auxiliary image which can be equated with the first image before increasing the decomposition level.