PURPOSE: To improve detection sensitivity, quicken response and thus to improve performance of a leakage breaker by detecting change in magnetic flux through inserting a hole element in the magnetic path of a zero-phase current transformer.

CONSTITUTION: A hole element 2 is inserted in the magnetic path of a magnetic core 1, and voltage is impressed by a DC power 23. Where difference occurs between currents flowing in reciprocating conductors L1, L2, a magnetic flux is produced according to the difference current and a hole voltage arises on the hole element 2, thus actuating a detection circuit 5.