To provide a controller which protects a neutral point resistor, etc., by breaking a current immediately in case that a large current flows in an earth transformer and continues to flow to the neutral point resistor, and enables easy replacement in case that the neutral point resistor is broken.

An automatic breaker 25 is connected in series between an earth transformer 7 and a neutral point resistor 21, and when it is informed that the this automatic breaker 25 has detected an earth current from an earth detector 15, it opens its own contact within thirty seconds. Moreover, when its own contact does not open, it compulsively opens the current path between the neutral point resistor 21 and an earth transformer 7 so as to break the earth current.