To make an alarm when a secondary circuit of a current transformer is open.

When a secondary circuit C2 of a current transformer CT is open due to a disconnection or the like, a resistance value becomes high and, by Ohm's law, a high voltage occurs across the secondary circuit C2. The high voltage is applied to a discharge part 1 and a resistor 2 connected in series, and the discharge part 1 becomes closed and a current runs through the resistor 2. When the current runs through the resistor 2, a potential difference occurs across the resistor. By the potential difference, a voltage relay 3 is operated. By the operation of the voltage relay 3, an alarm device 4 is energized and outputs an alarm indicating that the secondary circuit C2 is open.