A pattern having a plurality of bands in each of a plurality of lanes is photoelectrically converted, and the sum (histogram) of the density data (pixel data) of pixels lying in lines, each of which passes through a plurality of points in a direction perpendicular to the lanes, is detected on the basis of the density data of the respective pixels photoelectrically converted. Points representative of the existence regions of the respective lanes, for example the central positions of the respective lanes, are detected from the histogram, and the density data items of the pixels in the detected central positions of the respective lanes are collected to obtain the spectra of the respective lanes. The spectra are used to detect the plurality of maximum positions of each of them as band positions, whereby the band positions are automatically read.