There is disclosed a measuring method of illuminance unevenness of an exposure apparatus in which the illuminance unevenness resulting from a projection optical system, to project the light passed through the photomask onto the finite area on the photosensitive substrate via the projection optical system and to expose the photomask to the light, the method comprising calculating an average value of transmittance of the projection optical system of each path of the light emitted from one point of the photomask and reaching an imaging point for each of a plurality of imaging points in the finite area on the photosensitive substrate, and calculating the illuminance unevenness in the finite area on the photosensitive substrate from the average value of the transmittance obtained for each imaging point.