A data processing apparatus and an external storage apparatus arranged to transmit data by using serial signals and capable of reducing the number of signal lines for use to transmit serial data. A CLK line, a control line and a DT line are arranged between a data processing apparatus and a memory card. The DT line is a line through which data and commands formed into serial signals are bidirectionally transmitted. A synchronizing signal is transmitted to the CLK line. A control signal is supplied from the data processing apparatus to the memory card through the control line. In a period of time in which the signal level of the control signal is a high level, data or a command is transmitted to the DT line. In a period of time in which the signal level of the control signal is a low level, a status signal is transmitted to the DT line from the memory card.