We propose an all-sky-survey at hard X-ray region (up to 80 keV) by using a formation flight of two satellites. They consist of a telescope satellite, carrying a super mirror focusing at hard X-ray region, and a detector satellite, carrying scintillator deposited CCDs (SDCCDs) for hard X-ray region. These two satellites are in the same orbit (altitude is about 500 km) in formation flight. Since the super mirror will be a thin-foil mirror with poor imaging capability. Therefore, they control the separation of 20m±10 cm. Both satellites are in Keplerian orbit, then the viewing direction (from the detector satellite to the telescope satellite) scans along a large circle. Due to the precession of the orbit, the large circle gradually moves in the sky so that we can cover a large fraction of the sky. We can cover a large fraction of the sky without consuming a lot of fuel. However, the unseen regions are left near the ecliptic poles. This project, Formation Flight All Sky Telescope (FFAST), will be the first all sky survey at hard X-ray region.