By a technique different from conventional techniques, quality of an image obtained by nuclear medicine imaging is improved. In an image processing device, a first reconstruction process section generates, by a reconstruction process with respect to list mode data acquired by a PET device, an initial image indicating a concentration distribution of a radioactive substance. A scatter component derivation section derives, from the initial image, scatter component projection data indicating a scatter component of radiation rays by scatter estimation simulation. A second reconstruction process section generates, by a reconstruction process with respect to the scatter component projection data, a low resolution scatter image indicating the scatter component and having a resolution lower than that of the initial image. An upscaling process section generates an upscaled scatter image having a resolution identical with that of the initial image by upscaling the low resolution scatter image.