To provide a method for fault diagnosis of a small scale lightning protection apparatus which can make an accurate fault diagnosis of a nonlinear resistive element in a lightning protection apparatus comprising a series connection of a gap and a nonlinear resistive element without disabling a normal product.

The method for fault diagnosis of a lightning protection apparatus includes: a measurement step of applying a voltage not lower than a breakdown voltage of a gap 1 to a series-gapped test lightning protection apparatus A, and measuring an apparatus-passed transient current-time characteristic; a computation step of calculating a natural oscillation frequency of the test lightning protection apparatus A and a current peak value or strength at the natural oscillation frequency by a frequency analysis of the time characteristic; and an evaluation step of comparing the natural oscillation frequency of the test lightning protection apparatus A and the current peak value or strength at the natural oscillation frequency with those of a normal lightning protection apparatus, and diagnosing a fault from differences between them.