To detect annotation lines in medical image data. Horizontal annotation pixel determination means obtains the color component value difference between each pixel of a predetermined number of connected adjacent pixels in a first direction of the target pixel and an adjacent pixel thereof. If the total number of pixels having color component value differences, of the predetermined number of pixels is equal to or smaller than a first threshold, the horizontal annotation pixel determination means determines that the target pixel is an annotation pixel. If annotation pixels are successive in the horizontal direction in a predetermined number, horizontal annotation line determination means determines that the annotation pixels form an annotation line. The same applies to the vertical direction. The determined annotation lines are provided to border detection means.