1. A network setting device, comprising: a memory; and a processor which is connected to the memory and executes a process including: obtaining performance information indicating performance of a solver which solves a network design problem, the network design problem being designed as a mathematical programming problem in which a plurality of traffic demands which are to be simultaneously solved by the solver among a plurality of traffic demands for each section between given starting and ending sites are accommodated in a target network; determining an upper limit of a total number of design variables and an upper limit of a total number of constraints which indicate a problem scale in the network design problem based on the performance information; deriving a number of simultaneous demands which represents a number of the traffic demands so as to satisfy a condition that a number of design variables and a number of constraints which are to be provided to the solver are respectively equal to or less than the upper limit of the total number of design variables and the upper limit of the total number of constraints; executing the solver to perform a network design arithmetic operation for each of the derived numbers of simultaneous design demands; and setting accommodation of the traffic demands to the devices on the target network based on a result of the network design arithmetic operation.