To detect an isolated operation of a dispersed power supply inexpensively without influencing a distribution system.

A system for detecting an isolated operation of a dispersed power supply includes: a signal injection apparatus for, in a distributing substation for supplying power from a secondary side of a power transformer to a distribution line via a first breaker, injecting an AC signal of a predetermined frequency higher than a commercial supply frequency into a secondary zero phase circuit of the power transformer; and a signal detection apparatus for, in consumer equipment having the dispersed power supply connected to the distribution line via a second breaker, detecting a component of the predetermined frequency included in a zero phase voltage on the distribution line. The signal injection apparatus has an AC signal generation circuit for generating the AC signal, and a first voltage transformer for transforming the AC signal input into a primary side for injection into the zero phase circuit. The signal detection apparatus has a second voltage transformer for detecting the zero phase voltage, and a filter circuit for passing a frequency band including the predetermined frequency of the zero phase voltage.