Modification of software is made applicable while sustaining operation of a system even in case of a cluster system executing a job of high processing load. When modification data (3) is inputted, a scheduled modification application node determination means (1b) generates a list of scheduled modification application nodes (1c). Until the count of a number of node under application of modification counter (1f) indicative of the number of nodes under application of modification reaches an upper limit, a modification application node selection means (1e) extracts the node ID of nodes where a job is not operating sequentially from the list of scheduled modification application nodes as modification application node IDs. An operation management means (1g) stops operation of a modification application node corresponding to the modification application node ID and a modification means (1h) modifies a modification object software based on the modification data for a modification application node where operation is stopped.
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