A call unit transmits a call signal to verify the presence or absence of slave devices, which correspond to motors, to all of the slave devices after a master device is activated. A response reception unit determines the number of the slave devices, based on the number of response signals to the call signals. A communication speed change determination unit determines whether or not a data communication speed between the master device and the slave devices should be changed from a first speed to a second speed different from the first speed during a period from the time when the number of the slave devices is determined to the time when the drive of the motors is stopped, in accordance with a result of comparison of between the number of the slave devices and a predetermined number.