1. A golf swing-measuring system comprising: a computer for capturing a colored moving image obtained by photographing a golfer who swings by gripping a golf club, wherein said computer selects and extracts one or more images each showing a swing posture as check-point images, including an address image, a take-back shaft 8 o'clock image, a take-back shaft 9 o'clock image, a take-back unskillful arm horizontal image, a top image, a downswing unskillful arm horizontal image, a downswing shaft 9 o'clock image, an impact image, a follow-through shaft 3 o'clock image, and a finish image, and extracts from a large number of still images constituting said colored moving image, and obtains coordinates of positions of attention-focused points that operate in said check-point images when said golfer swings; wherein when a colored mark is provided on at least one of said attention-focused points, a search range which is a region in which presence of said colored mark is estimated is set in said check-point image, and a color range which is an allowable range in which color information of a pixel in said image is regarded as the same as that of a reference color of said colored mark is set, and a coordinate of a position of each of said attention-focused points is obtained in said search range by regarding a pixel whose color falls in said color range as a position of said color mark, wherein when a colored mark is not provided or when the provided colored mark is not found on the displayed image on at least one of said attention-focused points, differential processing is executed between said check-point image and a background image in which a golfer is not photographed to obtain a golfer's silhouette and extract a contour of said silhouette, a portion of said obtained contour where not less than a predetermined number of pixels having curvatures in a range from −10° to 10° are continuously present is extracted as a straight line portion, and a coordinate of a position of said attention-focused point is specified by using said straight line portion, and wherein a pixel present at a position where an extension of said straight line portion intersects with said contour is specified as a coordinate of a position of said attention-focused point.