When a system is activated, a host computer reads modules from an external nonvolatile memory, reconstructs control software for a machine in accordance with a system configuration information storage file, and transfers the control software to a volatile memory of each numerical control device. Various types of modules are stored collectively in the external nonvolatile memory without duplication. In updating the control software, it is necessary only that data be updated for the software modules in the external nonvolatile memory alone.