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 are specified characterized by comprising a relaxation solution derivation unit which derives a relaxation solution evaluation value that is the minimum value of a price of a system configuration of the computer system which achieves a part of the SLOs among a plurality of the SLOs and a search unit which searches for a parameter set of the design parameter by which all the plurality of SLOs are achieved and the price of the system configuration is made lowest by using the relaxation solution evaluation value.