Circumscribing pixels in the surroundings of an obtained image are obtained and the gradation levels of the circumscribing pixels are compared with a predefined threshold value. The number of circumscribing pixels of which the gradation level is smaller than the predefined threshold value is counted, and, if the number of circumscribing pixels, of which the gradation levels are relatively small, is larger than the number of circumscribing pixels in the surrounding of the image, the image is judged to be a low-key image. In contrast, if the number of circumscribing pixels, of which the gradation levels are relatively small, is smaller than the number of circumscribing pixels in the surrounding of the image, the image is determined to be an underexposed image.