The color of a measurement object, such as a vital tooth, and the color of a comparison object to be compared with the measurement object are displayed to a user in an easily comparable manner. The invention provides an image combining apparatus including an image extracting unit for extracting an image of a vital tooth (hereinafter referred to as “measurement-object image”) from a color image of the vital tooth and extracting an image of a shade guide (hereinafter referred to as “comparison-object image”) from a color image of the shade guide, a combining-reference-line setting unit for setting combining reference lines on the measurement-object image and the comparison-object image, a combining unit for combining the measurement-object image and the comparison-object image to form a combined image, and a correction unit for correcting at least one of the measurement-object image and the comparison-object image so as to align their outlines on the combining reference line in the combined image when the outlines are not aligned.