To realize a buffer device that selects a proper memory size automatically to minimize transmission delay.

Detection circuits 4, 5 detect a clock frequency on lines 2, 3, an arithmetic circuit 6 calculates a difference between both the frequencies and a switch 7 is used to select the capacity of a buffer memory 8 according to the above difference. Thus, an optimum memory capacity is set. A write circuit 9 writes data in the buffer memory 8 based on the clock of the line 2 and read-out circuit 10 reads data based on a clock on the line 3.