Since 2005, we have been developing "AIST-SOA Virtual Cluster Management System", which fosters utility computing on commercial data centers by employing virtualization technology. The system allows clients to reserve and configure a virtual cluster through an easy-to-use web interface. For each reservation, the system automatically allocates a bunch of virtualized computing resources including Xen/VMware based virtual CPUs, iSCSI storages, and networks for a virtual cluster, and installs and configures operating systems and applications by using Rocks, a cluster provisioning system. A prototype system has already been running, and we are now conducting research to improve stability and extensibility of the system. We also plan to apply our system to actual data centers.