1. A network design device, comprising: an adjustment unit to derive a number of simultaneous design demands so that a number of design variables and a number of constraints indicating a problem scale in a network design problem designed as a mathematical programming problem in which a plurality of traffic demands are accommodated in a target network for each section between given starting site and ending site are within an upper limit of the number of design variables and the number of constraints based on performance of a solver for solving the network design problem; and a design unit provided with the solver and to perform a network design arithmetic operation for each of the derived numbers of simultaneous design demands.