A maintenance computer system comprising a physical server connected through a switch, comprising: a switch configured to transfer received packet data for a destination based on a flow set to said switch itself; an integrated management apparatus configured to specify a maintenance object unit; and a controller configured to separate said maintenance object unit from said computer system by controlling setting of a flow to said switch or deletion of the flow set to said switch, wherein each of said destination and said maintenance object unit is defined by an IP (Internet Protocol) address or a MAC (Media Access Control) address, wherein said flow is defined by a rule and an action for processing said action to a packet matching with said rule, wherein when said maintenance object unit is a switch, said integrated management apparatus issues a route detour instruction to said controller, wherein said controller calculates a detour route which goes around said maintenance object unit in response to said route detour instruction, sets a flow corresponding to said detour route to the switch on said detour route, and deletes the flow corresponding to the communication route before detour set to the switch on said detour route, wherein when said maintenance object unit is a switch, said controller verifies whether or not said detour route can be calculated in response to the route detour instruction, wherein said integrated management apparatus instructs a VM (Virtual Machine) management apparatus to migrate a virtual machine which operates at the endpoint for which said detour route cannot be calculated by using a topology data from which said maintenance object unit is excluded, wherein said VM management apparatus migrates said virtual machine to the migration destination which is specified by said integrated management apparatus, and wherein said controller sets to said switch, the flow for transferring packet data for said virtual machine to said virtual machine of the migration destination. a switch configured to transfer received packet data for a destination based on a flow set to said switch itself; an integrated management apparatus configured to specify a maintenance object unit; and a controller configured to separate said maintenance object unit from said computer system by controlling setting of a flow to said switch or deletion of the flow set to said switch, wherein each of said destination and said maintenance object unit is defined by an IP (Internet Protocol) address or a MAC (Media Access Control) address, wherein said flow is defined by a rule and an action for processing said action to a packet matching with said rule, wherein when said maintenance object unit is a switch, said integrated management apparatus issues a route detour instruction to said controller, wherein said controller calculates a detour route which goes around said maintenance object unit in response to said route detour instruction, sets a flow corresponding to said detour route to the switch on said detour route, and deletes the flow corresponding to the communication route before detour set to the switch on said detour route, wherein when said maintenance object unit is a switch, said controller verifies whether or not said detour route can be calculated in response to the route detour instruction, wherein said integrated management apparatus instructs a VM (Virtual Machine) management apparatus to migrate a virtual machine which operates at the endpoint for which said detour route cannot be calculated by using a topology data from which said maintenance object unit is excluded, wherein said VM management apparatus migrates said virtual machine to the migration destination which is specified by said integrated management apparatus, and wherein said controller sets to said switch, the flow for transferring packet data for said virtual machine to said virtual machine of the migration destination.