1. An image retrieval method of a computer for retrieving similar objects, comprising: reading from a first database an image group mapped to an object as a retrieval target, and extracting an image feature quantity of a plurality of images of the read image group, the first database storing the object as the retrieval target and the image group including the plurality of images resulting from depicting the object as the retrieval target from viewpoints in different directions, with the object as the retrieval target mapped to the image group; reading from a second database an image group mapped to an object as a search key and extracting an image feature quantity of a plurality of images of the read image group, the second database storing the object as the search key and the image group including the plurality of images resulting from depicting the object as the search key from viewpoints in different directions, with the object as the search key mapped to the image group; determining a similarity between the image group mapped to the object as the search key and the image group mapped to the object as the retrieval target, based on the image feature quantity of the plurality of images of the image group mapped to the object as the retrieval target and the image feature quantity of the plurality of images of the image group mapped to the object as the search key; and displaying, on an output device, as retrieval results the plurality of images of the image group mapped to the object as the retrieval target in an order of similarity of the object as the retrieval target based on the determined similarity.