The Nb3Sn high magnetic field superconducting coil creates a strong magnetic field to confine the fusion plasma in the International Thermonuclear Reactor (ITER). Nb3Sn wire has excellent high magnetic field characteristics and productivity, but in the actual environment, it is greatly damaged by mechanical and thermal strains, and there is a concern that the critical current density (Jc) may deteriorate. When Zn was added as a solute element to the Cu–14 mass% Sn alloy used in the usual bronze method, the Zn equivalent became about 27 mass%, and the Nb3Sn phase was formed after the heat treatment. The strength of Nb3Sn wire rod is increased by the solid solution of Zn. Currently, as a further improvement, the addition of In, which is expected to be more solid solution strengthened than Zn, has been reported. Addition of In can be expected not only to strengthen the solid solution but also to improve the Jc characteristics. This study will investigate the effects of In addition to bronze using TEM and SEM microstructure observation of Nb3Sn filaments fabricated by heat treatment of Nb/Cu–10Sn–5In–0.3Ti (mass%) ultrafine multi–core wire.