An ROI coefficient and a non-ROI coefficient in first wavelet coefficient data corresponding to a first target image are determined on the basis of mask data which is developed for the first wavelet coefficient data. The ROI coefficient in the first wavelet coefficient data and a coefficient in second wavelet coefficient data corresponding to a second target image are synthesized. Synthesized coefficient data are thereby generated. Inverse wavelet transformation is performed on the synthesized coefficient data until a decomposition level becomes a predetermined end level. Synthetic image data are thereby generated.