A complier apparatus for a computer system that is capable of improving the hit rate of a cache memory is comprised of a prefetch target extraction device, a thread activation process insertion device, and a thread process creation device, and creates threads for performing prefetch and prepurge. Prefetch and prepurge threads created by this compiler apparatus perform prefetch and prepurge in parallel with the operation of the main program, by taking into consideration program priorities and the usage ratio of the cache memory.