A method for measuring a spherical aberration amount of a projection optical system that projects an image of a pattern formed on an original plate onto a substrate, includes: obtaining a first focal position in a direction of an optical axis of the projection optical system under a first measurement condition; obtaining a second focal position in the direction of the optical axis of the projection optical system under a second measurement condition; calculating the spherical aberration amount of the projection optical system based on a difference between the first focal position and the second focal position. Under the first measurement condition the focal position in the direction of the optical axis with respect to the spherical aberration amount does not change; and under the second measurement condition the focal position in the direction of the optical axis with respect to the spherical aberration amount changes.