When sending data, an ECU sends time lag information indicating a time lag that is a difference between time of generating time point at which the data is generated and time of sending start time point at which the ECU actually starts sending the data. When transferring the data, a relay apparatus adds time lag information indicating a time lag that is a difference between time of receiving start time point at which the relay apparatus starts receiving the data and time of transferring start time point at which the relay apparatus starts transferring the data, onto the time lag added on the received data. When receiving the data, an ECU determines for the received data the time of the generating time point that is earlier the time lag indicated by the time lag information of the received data than the time at which the ECU starts receiving the data.