PURPOSE: To make judgement of 3-dimensional undulation relations, etc. of two objects by moving moire interference fringes to the position where two sets of projection gratings have been relatively moved to form plural differential moire fringes and displaying these in different colors, brightnesses, codes, etc.

CONSTITUTION: When objects 3, 4 are projected from grating projectors 1, 2 via deformed gratings 5, 6, the moire interference fringe patterns are formed according to the undulations of the objects and are picked up in image pickup devices 7, 8. When the fringe pattern having been picked up by the device 8 via grating movers 14, 15 is moved to the position of the case when the relative distance between the gratings 5, 6 is changed and is processed with differential moire fringe generators 11 thru 13, then plural sets of differential moire fringes 18 thru 20 are formed. When these differential moire fringes 18 thru 20 are displayed in different colors, brightnesses, codes, etc. in a display device 21 and the various movements are controlled with the devices 14, 15, the differential moire fringes discriminated by the colors, etc. move on the device 11 and the 3-dimensional undulation relations between the two objects may be readily discriminated. In a similar manner, the discrimination of the change of the 3-dimensional undulations of one object with time may be accomplished.