The device manufacturing method includes a length measuring step (S5) of, on the basis of an observation target image of an SEM image taken from a direction having a predetermined angle from a direction perpendicular to a plane of a substrate, measuring the thickness of a target object, or the depth of etching, formed on the substrate. In addition, in the length measuring step, an etching angle made by a cross section of the etching and the direction perpendicular to the plane of the substrate is calculated from processing data of the target object, and the thickness of the target object or the depth of the etching is measured on the basis of the calculated etching angle.