1. A design method for an information processing apparatus for generating optimal LSI designs by automatic design space exploration, the method comprising: parsing, by the information processing apparatus, source code which describes a behavioral LSI to generate a parse tree; generating clusters based on the parse tree, each of the clusters including a group of source codes which can operate independently; executing a design space exploration of the clusters and generating a log file based on a structure and contents of the cluster; comparing a first log file of a previous cluster with a second log file of a current cluster, the first log file being obtained from a previous design space exploration, the second log file being generated from a current operation, and re-using a previous design space exploration result if the previous cluster has a same structure as that of the current cluster.