To predict correctly the change of thunder accident rate of a distribution line when a system configuration is changed.

An accident rate calculator 1 calculates the thunder accident rate 90 of a subsection area (unit mesh) which divided area through which the distribution line passes. Accident frequency calculators 2-1 to 2-n calculate the number of the annual thunder accidents 100 of unit meshes 1-n through which the distribution line passes by using the thunder accident rate 90. A distribution line accident frequency and accident rate calculator 3 acquires a number-of-annular thunder accidents 80 over the entire length of a distribution line from the total sum of the number of thunder accidents 100 of unit meshes 1-n, and acquires a thunder accident rate 81. An exact prediction can be performed by predicting the change of the thunder accident rate of the distribution line when the system configuration is changed by using the thunder accident rate of the distribution line at each unit mesh thus obtained.