In the present invention, in the photolithography process in which a certain focus condition has been already set, a film on a substrate is exposed to only zero-order light of a light source transmitted, and then developed to reduce a first portion of the film on the substrate. Further, the film on the substrate is exposed to zero-order light and higher order light of the light source transmitted, and then developed to reduce a second portion of the film on the substrate. Thereafter, the film thicknesses of the first portion and the second portion are measured, and the measured film thicknesses of the first portion and the second portion are converted into line widths of a resist pattern by previously obtained correlations between the film thicknesses and the line widths. The converted line width of the second portion is then subtracted from the converted line width of the first portion, whereby the line width depending only on the focus component is calculated. Based on the line width, a new focus condition is set.