A thin client environment providing system, comprising: a data center including at least one hypervisor and at least one layer 2 switch, said hypervisor being connected to said layer 2 switch, at least one virtual machine operating on said hypervisor, said layer 2 switch providing VLAN between the connected hypervisor at least one of said hypervisor being grouped as a hypervisor group; a server; and at least one terminal computer; wherein said server includes a VLAN group reservation unit adapted to, when receiving, from said terminal computer, a virtual machine addition request including an identifier of a tenant, information of a virtual operating system and an identifier of a generation destination hypervisor on which a virtual machine is to be added, obtain, from a data center data base which stores an identifier of the hypervisor, an identifier of a hypervisor group including the hypervisor and an identifier of the layer 2 switch connected to the hypervisor, so as to be correlated with each other, the identifier of the hypervisor group including said generation destination hypervisor and the identifier of the layer 2 switch connected to said generation destination hypervisor extract, from a tenant data base which stores an identifier of the tenant, an identifier of the hypervisor group, information of the virtual operating system, and an identifier of a VLAN connecting virtual machines related to a set of the tenant, the hypervisor group and the information of the virtual operating system, so as to be correlated with each other, the identifier of the VLAN for a set of the identifier of the hypervisor group obtained from said data center data base, the identifier of the tenant and information of the operating system included in said virtual machine addition request, and when no VLAN for the set exists, index a new identifier of a VLAN and register the indexed identifier of the VLAN for the set in said tenant data base to reserve the VLAN for the set, and a VLAN structure instructing unit adapted to, when for the layer 2 switch obtained from said data center data base none of the VLAN extracted or registered by said VLAN group reservation unit is registered in a switch data base which stores an identifier of said layer 2 switch and an identifier of said VLAN provided by the layer 2 switch, so as to be correlated with each other, register the identifier of the layer 2 switch and the identifier of the VLAN in said switch data base, so as to be correlated with each other, as well as transmit a request for structuring the VLAN on the layer 2 switch to the data center. a data center including at least one hypervisor and at least one layer 2 switch, said hypervisor being connected to said layer 2 switch, at least one virtual machine operating on said hypervisor, said layer 2 switch providing VLAN between the connected hypervisor at least one of said hypervisor being grouped as a hypervisor group; a server; and at least one terminal computer; wherein said server includes a VLAN group reservation unit adapted to, when receiving, from said terminal computer, a virtual machine addition request including an identifier of a tenant, information of a virtual operating system and an identifier of a generation destination hypervisor on which a virtual machine is to be added, obtain, from a data center data base which stores an identifier of the hypervisor, an identifier of a hypervisor group including the hypervisor and an identifier of the layer 2 switch connected to the hypervisor, so as to be correlated with each other, the identifier of the hypervisor group including said generation destination hypervisor and the identifier of the layer 2 switch connected to said generation destination hypervisor extract, from a tenant data base which stores an identifier of the tenant, an identifier of the hypervisor group, information of the virtual operating system, and an identifier of a VLAN connecting virtual machines related to a set of the tenant, the hypervisor group and the information of the virtual operating system, so as to be correlated with each other, the identifier of the VLAN for a set of the identifier of the hypervisor group obtained from said data center data base, the identifier of the tenant and information of the operating system included in said virtual machine addition request, and when no VLAN for the set exists, index a new identifier of a VLAN and register the indexed identifier of the VLAN for the set in said tenant data base to reserve the VLAN for the set, and a VLAN structure instructing unit adapted to, when for the layer 2 switch obtained from said data center data base none of the VLAN extracted or registered by said VLAN group reservation unit is registered in a switch data base which stores an identifier of said layer 2 switch and an identifier of said VLAN provided by the layer 2 switch, so as to be correlated with each other, register the identifier of the layer 2 switch and the identifier of the VLAN in said switch data base, so as to be correlated with each other, as well as transmit a request for structuring the VLAN on the layer 2 switch to the data center.