a light source configured to emit light;

an illumination optical system configured to illuminate the original with the light;

a substrate stage configured to hold the substrate and to be moved;

a projection optical system configured to project light from the original illuminated by the illumination optical system, the projection optical system and the substrate having a first gap therebetween that is filled with liquid, so that the substrate is exposed to light through the liquid, the projection optical system and the original;

a first detector configured to detect a light quantity of light emitted from the light source and passing along a light path of the illumination optical system;

a second detector disposed on the substrate stage and configured to detect a light quantity of light emitted from the light source and transmitted through the illumination optical system and the projection optical system; and

a calculator configured to determine a first conversion factor with respect to each of a plurality of exposure conditions, based on a first light quantity detected by the second detector in a first state where a second gap between the projection optical system and the second detector is filled with gas, a second light quantity detected by the first detector in synchronization with the detection of the first light quantity by the second detector, a third light quantity detected by the second detector in a second state where the second gap is filled with the liquid, and a fourth light quantity detected by the first detector in synchronization with the detection of the third light quantity by the second detector, the first conversion factor being used for conversion between a light quantity detected by the second detector in the first state and a light quantity detected by the second detector in the second state.