A method to plan a scan in consideration of past patient exposures. The method includes a step of sampling information on a patient exposure the subject has received during a scan performed for reconstructing tomographic images, which is appended to each of the reconstructed tomographic images, on the basis of identification information with which the subject is identified; a step of creating a distribution of patient exposures calculated relative to an axis orthogonal to the scanning directions on the basis of the sampled information on the patient exposure (an estimated patient exposure, which is estimated in planning a scan, and an exposure limit); and a step of displaying the created exposure distribution.