To provide a flashover detecting method and its device which can certainly detect the occurrence of flashover by removing the influence of flash light caused by a thunder or the like.

A flashover detecting device is equipped with a light receiving element 1 which receives light in the periphery of a conductor and converts the received light into an electric signal and outputs the electric signal as a light receiving signal; an alternating current coupler 2 to pass the fluctuating component of the light receiving signal from the light receiving element 1; and a detecting section (an amplifier 3, a constant current integrator 4, a judging unit 5) which receives the light receiving signal from the alternating current coupler 2 and promptly creates a detection signal to show a time when the light receiving signal fluctuates, and detects the flashover of the conductor on the interval of the fluctuation time of the detection signal.