A fringe pattern created by the interference of two laser beams at the measurement area of the laser-Compton beam-spot-size monitor invented at KEK to measure the size and shape of a nano-meter beam. By detecting laser photons scattered by beam particles incident on the fringe pattern, we can measure the beam spot. We have already succeeded in measuring a beam spot size of about 60 nano-meters (200 times the size of a typical atom). A next generation linear collider requires the spot size measurement of 3-nano-meter (10 times the size of a typical atom) beams. This can be realized by this technique.