When a load of a user is fluctuated, a data center dynamically changes resource allocation to the user according to the load and holds security for each user.A control program on a data center managing server creates a VLAN configuration table so as to allocate a user-dedicated VLAN including plural network switches for each user company, thereby configuring the ports of a load balancer and network switches allocated to a user to the user-dedicated VLAN. A VPN is configured from the user to the data center, whereby a VLAN tagging technique is used to hold security of the user from the user to the data center. The control program compares a user condition setting table created along the service level agreement for each user with the monitoring result of the computer operating state (the CPU utilization history in a VLAN operation table) to dynamically change the computer allocation and VLAN configuration at unsatisfied agreement. Specifically, the VLAN configuration table and the VLAN operation table are changed to change the VLAN configuration in the load balancer and the network switches (in the case of the Web layer). The control program changes the parameter (the sever allocation history in the VLAN operation table) related to charge information based on that.