Nb₃Sn high–field superconducting coil generates a strong magnetic field for confinement of a fusion plasma in an International Thermal Fusion Reactor (ITER). Nb₃Sn wire rods have excellent high magnetic field characteristics and good mass productivity, while in real environments, damage to the wire rod due to mechanical and thermal deformation is large, and deterioration of the critical current density (Jc) is a problem. It is reported that addition of In showed improvement in Jc as well as increase in strength by solid solution compared with addition of Zn. In this study, three types of wire rods, Nb/Cu–16Sn–0.3Ti, Nb/Cu 14Sn–2In–0.3Ti and Nb/Cu–10Sn–5In–0.3Ti, were prepared and the effect of In addition on the microstructure of Nb₃Sn filament was investigated．
