A measurement method for measuring a distortion of a projection optical system that projects a pattern, used by an exposure apparatus that exposes the reticle pattern onto an object to be exposed, the measurement method includes the steps of a first exposing step for exposing a mark pattern onto the object to be exposed, the mark pattern having a mark on or near an optical axis of the projection optical system and a mark beside the optical axis, and being arranged at a position of the reticle, a second exposing step for only exposing a mark on or near the optical axis of the projection optical system in the mark pattern, measuring step for measuring a shape of the mark formed on the object to be exposed via the first and second exposing steps, and calculating step for calculating the distortion of the projection optical system from the shape of the mark measured by the measuring step.