1. A device manufacturing method comprising: forming a pattern image by using a spatial light modulator to which an illumination light is irradiated, based on data corresponding to a device pattern; detecting the pattern image that has been formed; correcting the data corresponding to the device pattern by using a result of the detection, when quality of the pattern image varies from a predetermined specification; and exposing a substrate with a radiation image formed by the spatial light modulator based on the data that has been corrected, wherein the quality of the pattern image includes an absolute value of a line width error of the pattern image.