A communication system performs communication while switching between full-duplex communication and half-duplex communication. A slave device, which receives command packet signals requesting to write or read data from the master device, stores information in a response packet signal that specifies half-duplex communication in response to one of the received packet signals, and transmits the response packet signal to the master device, when the number of the received command packet signals has reached the maximum number of the command packet signals storable in a command signal queue.