An overlay measurement apparatus can judge whether an overlay displacement found from an image is accurate. It has a first measuring unit that captures images of first and second marks formed in different layers of a substrate and finds an overlay displacement between the two marks based on the images formed with light with a measurement wavelength range, a second measuring unit that captures the images of the two marks formed with light with an evaluation wavelength range different from the measurement wavelength range and finds the overlay displacement between the marks based on the images formed with the light with the evaluation wavelength range, and a calculation unit that calculates an index for accuracy of the overlay displacement in the measurement wavelength range based on the overlay displacements in the measurement and evaluation wavelength ranges found by the first and second measuring unit.