A two-dimensional look-up-table (LUT) storage unit stores a two-dimensional LUT in which each combination of two input values is associated with an output value for outputting a different color tone. A registration unit registers Positron Emission Tomography (PET) images of examination 1 and examination 2 by using X-ray CT images of the examination 1 and the examination 2 and an output-image creating unit acquires from the two-dimensional LUT an output value where input values are a combination of pixel values of corresponding pixels between the registered PET images, and creates an output image. Moreover, the output-image creating unit creates the output image from output values within a setting range specified on the two-dimensional LUT, among acquired output values. A display control unit controls display such that an output image or a composite image of an output image and an X-ray CT image created by a composite-image creating unit is to be displayed on a monitor.
