To properly detect leakage, even in a circuit that is large in ground resistance and is further large in capacitance to ground.

In a Δ-connected three-phase three-line system of circuit, this leakage detector detects a zero-phase current Io and information Pi about the zero-cross point of zero-phase current waveform from a zero-phase current transformer I, provided on a grounding conductor 10 and a current zero-cross point detecting circuit 8, and also detects information Pv about the voltage zero cross point of power voltage waveform between ungrounded two lines from a voltage zero-cross point detecting circuit 9, and inputs them into a microcomputer 5. The microcomputer 5 calculates the zero-phase current instantaneous value Io (Φ) at the phase 180° of power voltage waveform as a value equivalent to a grounding current, from the zero-phase current Io information, the current zero-cross point information Pi, and the voltage zero-cross point information Pv, and ground resistance Re value information inputted from an input device 2, and compares this value equivalent to the grounding current with a sensitiveness reference value Is which is computed, based on the zero-phase current effective value Iorms and the ground resistance Re value thereby judging the occurrence of leakage.