According to a flare measuring method in an embodiment, a reflective mask, in which one reflective coordinate in a slit direction in a mask surface is determined when one scanning coordinate is determined, is placed on a scanner that includes a reflective projection optical system. Moreover, a light intensity of the exposure light is measured by performing a scanning exposure on an illuminance sensor moved to a predetermined position in the slit direction in a slit imaging plane. Then, an amount of flare at an intra-slit position corresponding to a position of the illuminance sensor in the slit direction is calculated by using a light intensity of exposure light received from an intra-slit position that does not correspond to the position of the illuminance sensor in the slit direction in the exposure light.