The high performance of the superconducting joint of NbTi multifilament superconducting wires is achieved by using a sleeve that is reinforced and is tapered at the inner corner of the sleeve. The critical current of the joint is 1, 800A at 5T. This value is equal to the critical current of the superconducting wire. The superconducting joints are adopted as a superconducting magnet for synchrotron radiation. The critical currents of the joints between the wires for the coils are over 1, 100A at 5T for all joints, and those between the wires for the coil and persistent current switch exceed the quench current of the persistent current switch. By testing the superconducting joint using two filaments, it is suggested that the superconducting current passes at the joint when the interface of the NbTi filaments are slipped and deformed.