A data processing apparatus includes a data processing unit, a communication unit communicating with an external device via at least two logical lines including a first logical line and a second logical line having priority higher than the first logical line, a storage unit including a first buffer area storing data received by the communication unit via the first logical line and a second buffer area storing data received by the communication unit via the second logical line, and a control unit determining if the second buffer area stores data. According to determination that the second buffer area stores data, the control unit reads data from the second buffer area and controls the data processing unit to process the data read from the second buffer area, and according to determination that the second buffer area stores no data, the control unit determines if the first buffer area stores data.