1. A golf swing diagnosis system comprising a computer for capturing a colored moving image obtained by photographing a golfer who swings by gripping a golf club having colored marks attached to a shaft thereof, wherein said computer comprises: a means for converting said colored moving image into a plurality of still images; a means for executing binarization for each pixel of a plurality of said still images by using a specific threshold of color information and recognizing pixels, of said still images, which satisfy said threshold as positions of said colored marks so as to obtain coordinate data of each of said colored marks; an operation extraction means for recognizing a movement of said golf club shaft by using a movement vector amount of one of said colored marks computed based on said coordinate data of each of said colored marks or by using a vector angle between two of said colored marks; an image extraction means for selectively extracting still images necessary for diagnosing a golf swing from a plurality of said still images, based on data obtained by said operation extraction means; and an output means for outputting said extracted still image.