In a digital image processing, a weighting function having a pixel-to-pixel distance as a variable and a weighting function having a pixel-value difference of a neighboring pixel as a variable are each set (Step S4) for filtering with a bilateral filter. At this time, the weighting function applying weight depending on edge strength of a CT image (pixel-value difference between an adjacent pixel and a target pixel) is set using the CT image as another digital image without a PET image to be processed. As noted above, a filter coefficient is determined also with the CT image as the other digital image (Step S6). Then filtering is performed to the PET image as a digital image with the determined filter coefficient (Step S7). Consequently, filtering is obtainable with no influence of a noise level of the PET image, achieving maintenance of a space resolution and noise reduction.
