Flatness is an important parameter for various industrial products particularly silicon wafer, hard disk substrate, flat panel display, and other products. The large aperture flatness interferometer was developed by the AIST with the collaboration of FUJINON Company. The flatness of a specimen is compared with a precisely polished reference optical flat whose flatness has been calibrated with respect to the mathematical absolute flatness. The interferometer is capable of measuring flatness over a 300 mm diameter area with an uncertainty as low as 10 nm, which is the smallest uncertainty in the world.