Commands, which are stored in a queue within controllers in order of acquisition, are transmitted to mechanical I/F units (steps (1) and (2) in FIG. 7). A determination portion in each mechanical I/F unit determines whether or not the command is an internal command (for example, an error command) which is generated in the controllers. If the command is the internal command, the command is directly output to the mechanical controller. In contrast, if the command is not the internal command, a virtual mechanical controller outputs the command through synchronization processing for confirming whether the commands issued from the controllers are synchronized (steps (3) to (7) in FIG. 7).