1. An image orientation determination device comprising: an image features extraction section for extracting from an input image, image features indicative of a disposition feature of each element included in an image; an image features storage section for storing the image features of a positive training image, which is an image appropriate for image orientation determination, and the image features of a negative training image, which is an image inappropriate for image orientation determination; an orientation-specific features storage section for storing the orientation of the positive training image in relation to image features; an image similarity calculation section for calculating training image similarity indicative of the degree of similarity between the image features of the input image that is extracted by the image features extraction section and the image features of the positive training image and negative training image that are stored in the image features storage section; an image orientation determination decision section for deciding to determine the orientation of the input image when positive image similarity, which is the degree of training image similarity between the image features of the input image and the image features of the positive training image, is higher than a predetermined first threshold value, and deciding not to determine the orientation of the input image when negative image similarity, which is the degree of training image similarity between the image features of the input image and the image features of the negative training image, is not lower than a predetermined second threshold value; and an image orientation determination section for, when the image orientation determination decision section has decided to determine the orientation of the input image, calculating orientation similarity indicative of the degree of similarity between the image features of the input image and the image features stored in the orientation-specific features storage section, and determining the orientation of the input image in accordance with the orientation of the positive training image related to the image features stored in the orientation-specific features storage section when the calculated orientation similarity satisfies a predetermined condition.