To diagnose deterioration of a nonlinear resistance element of a lightning arrester, and to proactively prevent cracks of the nonlinear resistance element.

If a lightning current having a prescribed specification flows through the lightning arrester is assumed, if the maximum value of fracture energy Q shown by the product of a thermal stress σ and an elongation Δr in the radial direction of the nonlinear resistance element at that time exceeds a management value QL, it is discriminated that there is a possibility of generation of cracks in the nonlinear resistance element by the lightning current having prescribed specification values.