In a relay device, a communication unit receives a data frame transmitted cyclically through a communication line. A control unit stores, in a temporary storage unit, the data frame received by the communication unit. When the data frame stored in the temporary storage unit is transmitted, the control unit deletes the transmitted data frame from the temporary storage unit. When the control unit determines that, for the data frame received through the communication line, the amount of data relevant to the data frames stored in the temporary storage unit is equal to or greater than a threshold value, the communication unit transmits, through the communication line, a cycle change frame which gives an instruction to change a transmission cycle for transmitting the data frame to a longer cycle.