A method of measuring an illuminance distribution on a substrate; which is characterized in that the method comprises a step of reciprocatively moving a detecting sensor positioned on a substrate stage along a route during which the detecting sensor is intermittently stopped at a plurality of locations within an exposure region over the substrate, and, after reaching a turning location, moved to return along the route during which the detecting sensor is intermittently stopped again at these locations; in that measurement of illuminance is performed an even number of times at each location; and in that the illuminance distribution in the exposure region over the substrate is determined based not only on a measured value of illuminance obtained at each location during the forward movement of the detecting sensor but also on a measured value of illuminance obtained at each location during the returning movement of the detecting sensor.