The present invention relates to a measuring technique and especially relates to a method of accurately measuring the parallelism. The method is used to measure the relative parallelism which a flat surface of measured object M opposite to a reference plane W. Firstly, the measured object M is installed on a platform having rotating axis Z. The rotating axis Z is perpendicular to with the reference plane W A measuring apparatus comprise a optical source 1 and a camera 2 so that the lens 3 of the camera 2 aligns the side surface of said measured object to imaged a reflecting beam from which projected to the side surface of said measured object M A lateral coordinate axis u and a longitudinal coordinate axis u are a image's coordinate system of formed image. An origin of the coordinate system is defined at the center of the formed image. The measured point is the point that the higher or lower surface of said measured object intersect with its side face of throgh the longitudinal coordinate axis v When said measured object M rotated around the rotating axis, the parallel state of the surface of said measured object M was depict relatively that of the reference plane by detecting the position of image of measured point on the successive survey, so that determined non parallelism and adjusted position of said measured object M relative to the reference plane W. The invention can depict the parallel relationship between said measured object M with reference plane W and having high adjusting precision.