1. A similar case retrieval method comprising: receiving image features of partial images, the partial images including lesion portions; receiving information indicating a distribution of the lesion portions is a unilateral distribution in which the lesion portions are distributed unilaterally in either a right organ or a left organ; searching similar case data by searching through both case data for the unilateral distribution in the right organ and case data for the unilateral distribution in the left organ, based on the image features and image features of images contained in the both case data, outputting an image feature of an image contained in each of the similar case data being similar to at least one of the image features, wherein the both case data being included in a case database.