a method of measuring a position of a mark in a chamber in which a pressure inside the chamber is different from a pressure outside the chamber . the method includes a first detection step of detecting a reference mark fixedly disposed in the chamber , through a window arranged on the chamber , using a detector disposed outside the chamber , a second detection step of detecting the mark movable with respect to the reference mark in the chamber through the window using the detector , and 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 , so as to suppress a detection error caused by at least one of deformations of the chamber and the window , in which at least one deformation is due to a difference between the pressure inside the chamber and the pressure outside the chamber .