An exposure apparatus includes an illumination optical system for illuminating a reticle with exposure light, a projection optical system for projecting a pattern of the reticle onto a plate via a liquid supplied to a space between the projection optical system and the plate, a supply pipe for supplying the liquid to the space, a recovery pipe for recovering the liquid from the space, and a measuring unit for measuring a refractive index of the liquid. The measuring unit includes (i) a light source for generating a measurement light having the same wavelength as that of the exposure light, (ii) a liquid reservoir for storing the liquid, the liquid reservoir being disposed apart from the space via the supply pipe or the recovery pipe, and having a transmitting surface for transmitting the measurement light and a reflecting surface for reflecting the measurement light transmitted by the transmitting surface and the liquid in the reservoir, and (iii) a detector for detecting the measurement light reflected on the reflecting surface and transmitted by the liquid in the liquid reservoir and the transmitting surface.