To provide a tide calculation method and a tide calculation device for looped system that can calculate a node voltage distribution and a line tide distribution using a tide calculation method for radial power distribution system by placing an interactive switch at a loop point in an open state and connecting virtual power supply models to both ends thereof.

The virtual power supply models are connected to both the ends of the interactive switch placed in the open state, and the radial power distribution system is replaced with the looped power distribution system so that a node of each power distribution line on a power supply side is a head node and a node on a virtual power supply model side is an end node. Active power and reactive power of the virtual power supply models such that the voltage across and the phase angle of the interactive switch are equal respectively are calculated, and the node voltage distribution and line tide distribution are calculated by the tide calculation method for radial power distribution system using at least the voltage and phase angle of the head node, the voltage and phase angle of the end node, and the active power and reactive power.