A data transfer rate deciding method and circuit that can decide, upon reception of a data packet, whether a data transfer rate decided by a reception side is the same as that of data transferred by a transmission side. The method includes identifying a data transfer rate by a procedure performed before data transfer; regenerating a reception clock pulse based on a received signal upon data reception; comparing the reception clock pulse to a reference frequency clock pulse given on the basis of the data transfer rate identified by the procedure; and notifying as an error if the data transfer rate identified by the procedure is different from that of the received data.