Abstract: In cases where a first communication control unit locks a second communication control unit to communicate with the second unit, the first unit stores a data representing a time remaining until the locking of the second unit is released. In these cases, the second unit stores a data representing an address of the first unit as a lock address corresponding to an address of a unit which locks the second unit. In cases where a third communication control unit tries to communicate with the second unit and fails the communication with the second unit, the third unit connects with the second unit and reads out the data representative of the lock address from the second unit, and then the third unit connects with the first unit in response to the lock address and reads out the data representative of the lock remaining time from the first unit. The third unit retries to communicate with the second unit after the lock remaining time elapses.