To prevent the influence of external vibration, to achieve a reference plate support structure, and to reduce manufacturing costs by providing a phase difference varying means for changing the phase difference of luminous flux passing two paths for fringe scanning at a path match path section.

An optical path length changing section 30 fixes total reflection mirrors 23, 24 on a second path on a plate 31, and drives a piezo element 14 mounted to the plate 31, so that the total reflection mirrors 23, 24 are reciprocated in the direction of an arrow A in one piece and the phase difference between light fluxes passing two paths is changed. The piezo element 14 is driven by a piezo driver 16 as a drive means. The piezo driver 16 outputs a specified drive voltage to the piezo element 14, based on a piezo element drive signal outputted from a stage controller 15 according to a control signal from a computer 11.