In a wavefront sensor, an optical wavefront to be measured is split into a first optical path and a second optical path. A wavefront W1 in the first optical path is transmitted through a first compensation member 7, and a wavefront W2 in the second optical path is transmitted through a second compensation member 8. Wavefronts W1 and W2 are mixed together by a semi-transparent mirror 6 with the wavefronts being displaced from each other by a shearing quantity S to form an interference fringe. An optical path difference that occurs between two wavefronts W1′ and W2′ which reach the interference measurement plane M in a state where the wavefronts are inclined due to the arrival direction of the optical wavefront to be measured is compensated when the wavefronts W1′ and W2′ are transmitted through the first and second optical path difference compensation members 7 and 8, respectively.