Abstract: Herein disclosed is a disk controller having a cache. This cache is so managed that a predetermined part composed of at least one of a plurality parts divided therefrom is initialized. In response to a request for cache activation from a host computer, the initializations of the cache are partly executed in a repeated manner, and a request for disk input/output is intermittently processed so that the whole initializations of a cache having a large capacity can be executed in parallel with the online process.Moreover, the logic failure of the cache and the hardware failure of a memory are divided. When the logic failure occurs, the use of the case is temporarily prohibited, and the whole initializations of the cache are executed automatically at the side of the disk controller without any intervention of a maintenance man so that the cache can be used again.