A determination target region includes a combination of a red gray image, a green gray image, and a blue gray image. A CPU part transforms a three gray value matrixes to a single first data array uniquely associated with a pixel position in the determination target region. The CPU part calculates a normalized correlation value with a single second data array transformed from a model image according to the same predetermined rule, and determines whether or not a match is found with the model image according to whether or not the normalized correlation value exceeds a predetermined threshold value.