In a data communication system that performs data communication between a master station and slave stations in a token passing system employing an Ethernet (registered trademark), when a communication processing unit of a master station subjects processing for circulating a token frame between the master station and slave stations and receiving data from the slave stations to retry processing, the communication processing unit subjects the processing to the retry processing without transferring data received from the slave stations to a data processing unit that performs transmission and reception of data between the data processing unit and an arithmetic processing device.