To provide a phase sequence determination method for a distribution line to facilitate its management.

A master station and a plurality of slave stations located in a high-tension distribution system are provided. The master station performs data transmission/reception to/from the plurality of slave stations according to an IP protocol. When determining phase sequence on a distribution line, the master station designates arbitrary two slave stations among the plurality of slave stations as a reference slave station and as a determination slave station, and issues a command for requiring phase sequence determination data on step S1. Then, on step S2, zero-crossing detection data are received from the reference slave station and from the determination slave station to record the subject matter of the detection data and data reception time. On step S3, a phase sequence determination process is done according to the subject matter of the detection data and the data reception time. In the determination process, reference time of phase difference in the phase sequence, allowable error in in-phase discrimination, etc., are used as conditions for phase sequence determination.