An islanded power system with a distributed power supply includes a system abnormality detection unit, which outputs an abnormality detection signal when an abnormality in a commercial system is detected; a breaker, which interrupts a supply of power from the commercial system when the abnormality detection signal is input from the system abnormality detection unit; a status detection unit, which outputs a breaker status signal when the breaker's open state is detected; and an electrical storage device, which compensates a fluctuation in load until a rotating machine generator switches to an islanded operation after the supply of power from the commercial system is interrupted by the breaker by starting the supply of power when the breaker status signal is input.