To provide an insulation abnormality detector that prevents deterioration in accuracy due to a noise effect in determining insulation abnormality between a high voltage battery and a body grounding.

An insulation abnormality detector includes: a serial connection circuit of a coupling capacitor 101, a detection resistor 102, and an oscillator 103, which is connected to a minus pole of a high voltage side battery, an observation signal V2 outputted from a connecting portion of the coupling capacitor 101 and the detection resistor 102 being inputted to a notch filter 104 and a subtraction circuit 105; the notch filter 104 for inputting the observation signal V2 and cutting off only a component in the vicinity of a frequency of an oscillation signal V1 oscillated by the oscillator 103, an output signal Vm of the notch filter 104 being inputted to the subtraction circuit 105; the subtraction circuit 105 for performing subtraction processing between the observation signal V2 and the output signal Vm of the notch filter 104; and a determination circuit 106 for determining insulation abnormality by comparing a waveform of an output signal Ve of the subtraction circuit 105 and a waveform of the oscillation signal V1 of the oscillator 103.