A PET-MRI device according to an embodiment includes image generators and a derivation unit. The image generators capture an image of a target placed in an effective visual field of a PET by the PET and an MRI so as to generate a PET image and an MR image. The derivation unit calculates a strain correction factor for correcting strain on the MR image based on a positional relation between a target that is expressed on the PET image and a target that is expressed on the MR image.