A phase split structure of a superconducting cable includes three cable cores (102) each having a shield layer (203) provided around a superconductor, a splitter box (1) housing the three cable cores (102) extending from an assembly portion where the three cable cores (102) are assembled into the cable, in a state in which the cable cores (102) are spaced apart from each other, and a shield connecting portion (2) connecting respective shield layers of the cable cores (102) to each other within the splitter box (1). The shield connecting portion allows the cable cores to have their respective shield layers connected together with low resistance and each shield layer can pass a current substantially equal in magnitude to that which each superconductor passes. Thus in each shield layer a magnetic field can be formed having a level that can cancel a magnetic field generated from each superconductor. The structure can thus effectively prevent a large magnetic field external to the cable core.