A lightning surge detector comprising: a first terminal unit connectable to a ground-side terminal unit of a surge protective device; a second terminal unit connectable with a ground wire; a conducting coupling bar adapted to couple the first terminal unit and the second terminal unit; a detection coil placed in a vicinity of the conducting coupling bar and adapted to output an induction current induced by a lightning surge current flowing through the conducting coupling bar; a waveform processing unit adapted to convert the induction current into a voltage, thereby generating a detection voltage, perform a stretching process on a voltage waveform of the detection voltage along a time axis, and thereby output a waveform-modified voltage; a computation control unit adapted to accept input of the waveform-modified voltage, calculate a current value of the lightning surge current from a voltage value of the waveform-modified voltage, and produce a lightning surge current calculation result; and a display unit adapted to display the lightning surge current calculation result so as to be viewable from outside under the control of the computation control unit, characterized in that the first terminal unit, the second terminal unit, the conducting coupling bar, the detection coil, the waveform processing unit, the computation control unit, and the display unit are unitized by being housed in a casing, the detection coil comprises: a core substantially U-shaped in cross section and arranged so as to embrace the conducting coupling bar, and a pair of a first winding and a second winding wound around the core so as to be symmetrical to each other with respect to the conducting coupling bar; and the first winding and the second winding are wound in a predetermined direction and connected in series under a predetermined connection condition such that the induction current will not flow when an external magnetic field is applied, and that the induction current will be outputted when the lightning surge current flows through the conducting coupling bar. a first terminal unit connectable to a ground-side terminal unit of a surge protective device; a second terminal unit connectable with a ground wire; a conducting coupling bar adapted to couple the first terminal unit and the second terminal unit; a detection coil placed in a vicinity of the conducting coupling bar and adapted to output an induction current induced by a lightning surge current flowing through the conducting coupling bar; a waveform processing unit adapted to convert the induction current into a voltage, thereby generating a detection voltage, perform a stretching process on a voltage waveform of the detection voltage along a time axis, and thereby output a waveform-modified voltage; a computation control unit adapted to accept input of the waveform-modified voltage, calculate a current value of the lightning surge current from a voltage value of the waveform-modified voltage, and produce a lightning surge current calculation result; and a display unit adapted to display the lightning surge current calculation result so as to be viewable from outside under the control of the computation control unit, the first terminal unit, the second terminal unit, the conducting coupling bar, the detection coil, the waveform processing unit, the computation control unit, and the display unit are unitized by being housed in a casing, the detection coil comprises: a core substantially U-shaped in cross section and arranged so as to embrace the conducting coupling bar, and a pair of a first winding and a second winding wound around the core so as to be symmetrical to each other with respect to the conducting coupling bar; and the first winding and the second winding are wound in a predetermined direction and connected in series under a predetermined connection condition such that the induction current will not flow when an external magnetic field is applied, and that the induction current will be outputted when the lightning surge current flows through the conducting coupling bar. a core substantially U-shaped in cross section and arranged so as to embrace the conducting coupling bar, and a pair of a first winding and a second winding wound around the core so as to be symmetrical to each other with respect to the conducting coupling bar; and the first winding and the second winding are wound in a predetermined direction and connected in series under a predetermined connection condition such that the induction current will not flow when an external magnetic field is applied, and that the induction current will be outputted when the lightning surge current flows through the conducting coupling bar.