Abstract: A plurality of interference beams are obtained by making a pair of beams of reference and test-sample light diverge into a plurality of optical paths; providing phase difference between reference and test-sample beams constituting each of a plurality of pairs of reference and test-sample beams having traveled on the plurality of optical paths. The plurality of interference beams are respectively received by light receiving means such as image pickup elements, and the phase difference between the beams of reference and test-sample light is obtained on the basis of signals relating to the plurality of interference beams supplied from the light receiving means, thus obtaining the state of a wave surface of the test-sample beam.