To surely determine an optimal position of an optical element being used for adjusting the difference of optical path in the low coherent interference.

The method for measuring interference measures the transmission wave front of a lens to be tested by using an interferometer having an optical system, in which a light beam to be detected that is one passing through the lens to be tested interferes with a prescribed reference light beam and having a detecting means which detects an interference fringe generated by the interference. In the interferometer, an optical element is moved, and a variation curve of the contrast of interference fringe indicated by the detecting means is formed at each position of the interference fringe during the movement, and the position which produces the maximum contrast in an overlapping area of the curve formed by above measuring procedure is obtained, and the position is determined as the optimal position of the optical element when carrying out the measurement.