A path pair designing method is provided which enables a sure creation of a path pair so long as a network having a configuration that allows the path pair to be created is employed. The path pair designing method includes a first step of producing an objective function to be used as a reference for selection of the path pair, a second step of producing a first constraint expression to be used as a condition required for acquiring the path pair, a third step of producing a second constraint expression to be used as a condition required to put each of communication paths making up the path pair into a disjoint state, and a fourth step of solving a mathematical programming problem made up of the objective function produced in the first step, the first constraint expressions produced in the second step and the second constraint expressions produced in the third step, and of selecting the disjoint path pair.