A 20T large bore superconducting magnet has been developed at the National Research Institute for Metals. In this paper, design for its coils is mainly discussed. Design policy for this magnet system is to meet two contradicting requirements; large bore diameters and generation of high magnetic fields. The magnet consists of four coils, and should be operated at 1.8K in saturated superfluid helium to generate its highest field. Magnetic fields of 15T in a 314mmφ thermally insulated chamber, of 18T in a 160mmφ bore and of over 20T in a 44mmφ bore are designed to be generated with combinations of these four coils.
