1. An image processing apparatus for processing an image composed of an effective area as a meaningful area and an ineffective area as a remaining area, comprising: left coordinate position detecting means for detecting a coordinate position of a left-end pixel of pixels that are determined to be out of the ineffective area from among pixels arranged in a horizontal line in the image by comparing a pixel value of each pixel with a first threshold value; right coordinate position detecting means for detecting a coordinate position of a right-end pixel of pixels that are determined to be out of the ineffective area from among pixels arranged in a horizontal line in the image by comparing a pixel value of each pixel with the first threshold value; upper coordinate position detecting means for detecting a coordinate position of an upper-end pixel of pixels that are determined to be out of the ineffective area from among pixels arranged in a vertical line in the image by comparing a pixel value of each pixel with the first threshold value; and lower coordinate position detecting means for detecting a coordinate position of a lower-end pixel of pixels that are determined to be out of the ineffective area from among pixels arranged in a vertical line in the image by comparing a pixel value of each pixel with the first threshold value.