1. A non-transitory computer readable storage medium having stored therein a software update management program for causing a computer to execute a software update process for a disk node in a distributed storage system which performs data management with redundancy with multiple disk nodes, said software update management program causing said computer to function as: migration content judgment unit which reads a pre-migration environment table in which a software name and a version number of software before being updated in a target disk node which is an update target have been set for each function of said target disk node, and a post-migration environment table in which the software name and the version number of the software after being updated in said target disk node have been set for each function of said target disk node, from a memory device, and judges a migration content indicating a transition of the software name and the version number between before and after the update, for each function of said target disk node; migration pattern determination unit which reads a migration pattern determination table in which a correspondence relationship between multiple migration patterns and migration contents to be applied with the migration patterns has been set, from the memory device, and determines a migration pattern to be applied to said target disk node based on the migration content judged by said migration content judgment means and said migration pattern determination table, said multiple migration patterns being classified into a first class migration pattern in which an update of data managed by said target disk node is prohibited and thereby a software update is performed while data redundancy is maintained, and a second class migration pattern in which the data managed by said target disk node is copied to another one of said disk nodes and thereby the software update is performed while the data redundancy is maintained; and update unit which reads said post-migration environment table from the memory device, issues an instruction to said target disk node according to the migration pattern determined by said migration pattern determination means, and updates software which executes each function of said target disk node, to the software name and the version number which have been set in said post-migration environment table.