1. An optimal design assistance device which assists in design of controllable design parameters of a computer system to which a plurality of Service Level Objects (SLOs) defining non-functional requirements of the computer system are specified, the optimal design assistance device comprising: at least one memory configured to store a program and at least one processor configured to read the program to execute: a derivation unit that derives a minimum value of a price of a system configuration of the computer system, as a relaxation solution evaluation value, by using a part of the SLOs, the relaxation value being determined at high speed without simulation; a search unit that searches for a parameter set of design parameters using the derived minimum value of the price, which achieves all of said plurality of SLOs and minimizes the price of the system configuration; and an optimality assessment unit that compares the relaxation solution evaluation value with the minimum value of the price of the system configuration set in advance and makes the search unit search for the parameter set when the relaxation solution evaluation value is smaller than the minimum value of the price of the system configuration set in advance.