A communication system includes a master device and a slave device that perform, at predetermined periods, communication for sharing storage data stored in a cyclic memory. The master device transmits communication data that includes (i) identification information distinguishing a period relating to the communication data from other periods before and after the period and (ii) target information indicating whether the communication data is a target that is to be communicated by the communication at the predetermined periods or is an exception to be excluded from being the target. The slave device discards the received communication data when (i) the target information included in the received communication data indicates that the communication data is the target and (ii) a period indicated by the identification information included in the communication data is different from a current period, and processes the communication data.