A method of measuring a position of a mark in a chamber, a pressure inside the chamber being different from a pressure outside the chamber. The method includes a first detection step of illuminating a reference mark fixedly disposed in the chamber using a first illuminator disposed inside the chamber, and detecting the reference mark using a detector disposed outside the chamber. The method also includes a second detection step of illuminating a mark, which is movable with respect to the reference mark in the chamber, using a second illuminator disposed inside the chamber, and detecting the mark using the detector. The method further includes a calculation step of calculating a position of the mark with respect to the reference mark based on detection results of the first and second detection steps.