A method comprises determining an exposure condition by executing a process including computing an image formed on an image plane under the current exposure condition while changing the exposure condition, and evaluating a line width of the computed image, and exposing the substrate under the determined exposure condition, wherein the determining includes, computing a simplified evaluation value of the computed image, changing the exposure condition and executing the process in the changed exposure condition, after evaluating the computed image if the simplified evaluation value satisfies an allowable value, and changing the exposure condition and executing the process in the changed exposure condition without evaluating the computed image if the simplified evaluation value does not satisfy the allowable value.