A method for determining the location and density of bands of biopolymers which are formed by developing and resolving a mixture of biopolymers on a supporting medium in one direction to form a lane of plural bands. The method includes the steps of inputting an image of the lane to a computer in the form of digital data comprising information of location and density, displaying the image, and setting a belt area along the lane which encompasses the aimed bands of the biopolymer. The belt area is then scanned a plurality of times in parallel directions crossing the lane with sufficiently small spacing so as to scan every band present within the belt area, in order to detect relationships of location and density on the digital data in the direction of the lane within the belt area. One dimensional data is then prepared to enable a determination of the location and density of the aimed bands.