To provide a technology for efficiently advancing design work of an inner cable.

When, for example, a rupture is caused with an inner peripheral part of a covering layer as a starting point, as a result of an endurance test for evaluating the durability of the inner cable of curving along a guide pulley, the design value of the inner cable or the guide pulley is changed so as to reduce maximum stress generated in an inner peripheral part of the covering layer. After changing the design value, the fact that the maximum stress generating is not increased is confirmed even on the outer peripheral part of the covering layer from which the rupture starts. After that, the endurance test is performed again, and the durability of the covering layer is confirmed.