A pixel-value detecting circuit (1) detects RGB values of each pixel of an input image and outputs the detected RGB values to a connection weight determining circuit (2). When both of two adjacent pixels have achromatic color, the connection weight determining circuit (2) determines a first connection weight, which is a connection weight between the two pixels, by using only the RGB values. When one of the two pixels has achromatic color, the connection weight determining circuit (2) determines, as a connection weight, a second connection weight that is smaller than the first connection weight, by using the RGB values and saturations. When both of the two pixels have chromatic color, the connection weight determining circuit (2) determines, as a connection weight, a third connection weight that is greater than or equal to the first connection weight, by using the RGB values and hues. An image segmenting circuit (3) segments the input image into images of subjects by using the connection weight determined by the connection weight determining circuit (2).