1. A maintenance method of a disk controller connected between a host computer and a disk drive and having a plurality of processors for controlling data transfer between said host computer and said disk drive, comprising the steps of: dividing said processors into a plurality of clusters; shifting a cluster mode from a cluster non-restricted mode in which distributed processing of a job among clusters belonging to any clusters is permitted to a cluster restricted mode in which a cluster to which processors permitted to distributedly process a job belong is restricted; selecting at least one cluster of said plurality of processors as a cluster to be maintained; versioning up programs to be executed by the processors belonging to said cluster to be maintained; and executing the version-up programs by a new program by the processors belonging to said cluster to be maintained.