Provided are a diagnosis assisting device, an imaging processing method in the diagnosis assisting device, and a non-transitory storage medium having stored therein a program that facilitate a grasp of a difference in an diseased area to perform a highly precise diagnosis assistance. According to an image processing method in a diagnosis assisting device that diagnoses lesions from a picked-up image, a reference image corresponding to a known first picked-up image relating to lesions is registered in a database, and when a diagnosis assistance is performed by comparing a query image corresponding to an unknown second picked-up image relating to lesions with the reference image in the database, a reference image is created from the reference image by geometric transformation, or a query image is created from the query image by geometric transformation.