A computer system of the present invention is provided with an open flow controller 3, and a switch 4i. The switch 4i notifies a MAC address contained in packet data to the open flow controller 3 when the packet data from the virtual machine that the migration has completed does not fit with a rule shown by the flow set to the switch itself. The open flow controller 3 sets a communication flow for a migration destination VM generated according to the notified MAC address to the switch 4i. The switch 4i transfers the packet data for said virtual machine which follows a rule 444 shown by the communication flow for said migration destination VM, to the migration destination virtual machine based on an action 445 shown by the communication flow for said migration destination VM.