Standard image data representing standard image including image of slit of a standard device is produced by photographing the standard device including the slit which includes a standard slit segment having a predetermined width. A width of the standard slit segment in the standard image is determined by analyzing the standard image data. Gap image data representing a gap image which is an image including an image of a discharge gap of a spark plug is produced by photographing the spark plug. A distance of the discharge gap in the gap image is determined by analyzing the gap image data. By using the distance of the discharge gap in the gap image and the width of the standard slit segment in the standard image, it is determined whether an actual distance of the discharge gap is within a predetermined allowable range or not.