A frame is formed by forming in a base having at least three through-holes 2, 3 and 4 in the same cross section cavities 6 and 7 crossing said through-holes 2 to 5 at a given distance. A cell is formed by sealing an area sandwiched by said two cavities 6 and 7 with translucent sheets 8 and 9. Interference fringes are generated by sending two beams from a light-source element 19 into the cell through as parallel plane mirror 14 and reflecting the beams from the cell to a single spot on the parallel plane mirror 14 through a prism 15.