To transmit data from a master station to slave stations through broadcasting when there is a request providing a received data switching signal and a broadcast data signal for showing broadcast transmission to the plural slave stations by the master station and switching the slave station to the broadcast data reception according to the received data switch signal.

The output signal of a master station 1 is provided with a broadcast data signal BD and a received data switching signal BC in addition to conventional identification data signal SD-1, data clock signal SK and reception request signal ST. When transmitting the broadcast data, the master station 1 turns the signal BC into broadcast data signal selecting state and outputs it to plural slave stations 2 together with the signals BD and SK. At the slave station 2, a received data switch circuit 21 selects the signal BD according to the signal BC and the broadcast data are outputted through a received data register 22 to a processor 23. Thus, when thereis the request, all the slave stations 2 can simultaneously receive the broadcast data transmitted from the master station 1.