Abstract: A communication system includes at least one slave communication device to be a communication partner to a master communication device for performing data transfer therebetween using communication packets. Each of the communication packets includes control bits which are information relating to locking for the slave communication device and data length bits indicative of the size of a message to be transferred. In transmitting the communication packet from the master communication device to the slave communication device. The slave communication device is locked when locking is designated by the control bits and is unlocked when a message having a data length specified by the data length bits has been transferred successfully. Thus. The slave communication device can be locked or unlocked by the master communication device using the same control bits in accordance with information as to whether or not a predetermined message has been transferred successfully.