FORCE is a 1.2 tonnes small mission dedicated for wide-band fine-imaging x-ray observation. It covers from 1 to 80 keV with a good angular resolution of $15$ half-power-diameter. It is proposed to be launched around mid-2020s and designed to reach a limiting sensitivity as good as $F_X (10-40~{rm keV}) = 3 times 10^{-15}$~erg cm$^{-2}$ s$^{-1}$ keV$^{-1}$ within 1~Ms. This number is one order of magnitude better than current best one. With its high-sensitivity wide-band coverage, FORCE will probe the new science field of missing BHs, searching for families of black holes of which populations and evolutions are not well known. Other point-source and diffuse-source sciences are also considered. FORCE will also provide the hard x-ray coverage to forthcoming large soft x-ray observatories.