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Nb3Sn superconductors made by the powder-in-tube (PIT) process have been developed for application to particle accelerator magnets, high field magnets and many other fields. Recently, newly developed PIT process wires having the Ta-Sn core/Nb(Nb-Ta) sheath composite have aroused interest since these wires show excellent high field performance up to 23 T at 4.2 K. We found that the superconducting characteristics of the PIT process wires were improved by locating Nb-Ta rods in the Ta-Sn core. It is known as the RIC (rod-in-core) process. Multifilamentary wires fabricated by the RIC process using a Cu stabilizer were studied. To improve critical current density (Jc) in RIC wires, different sheath materials were studied. The wires using Ta sheath and Cu stabilizer have shown the highest Jc performance, though no reacted superconducting layer existed at the interface between the core and the sheath. We investigated the composition and the microstructure of Nb3Sn layer in the wires with Nb and Ta sheath. In this report we discuss the origin of high Jc in the Ta sheathed RIC wires through the composition and microstructure analysis.