1. An exposure method in which overlay exposure to a photosensitive object is performed by overlaying layers on the object, the method comprising: a selection process in which a reference layer on the object that serves as a reference to obtain information on overlay exposure of a next layer on the object is selected for a first axis and a second axis, respectively, of a two-dimensional orthogonal coordinate system that serves as a reference on overlay, from among at least two layers on the object that have been already exposed, wherein, the information on overlay exposure includes a distortion component of a projected image at the point in time when exposure of the selected reference layer was performed, in a projection exposure apparatus that was used for the exposure of the selected reference layer, the selection process includes selecting a plurality of reference layers on the object for at least one of the first axis and the second axis so that a reference layer selected for the first axis and a reference layer selected for the second axis are different, and the distortion component of the projected image of the axis for which the plurality of reference layers have been selected is computed by computation processing using distortion components of projected images of the selected plurality of reference layers, wherein the computation processing is weighted average processing.