A data processor capable of preventing a system from starting up improperly due to incomplete program updating. The data processor receives, by way of an external interface, a program which is sent by communicating medium such as a broadcast wave or a telephone network from an external device and stores the received program into a memory B which is different from a memory A storing a program for the system when a version of the received program is newer than that of a program currently executed by the data processor, thereby starting up an MPU at the next time with the program stored in the memory B.