Provided is a data processing device that can prevent data used by a program from being used by another program in an unauthorized manner, regardless of the quality of the programs. The data processing device includes: a CPU 0201 for executing programs; and an unauthorized operation prevention circuit 0105 that prevents unauthorized accesses to data between programs. An unauthorized operation prevention control unit 0106, which operates in the protected mode and controls the circuit 0105, judges whether or not to permit a program B 0103 that runs in the normal mode to use a memory area that is used by a program A 0102 that runs in the normal mode, based on a function flag assigned to the program B 0103. If it judges to permit, the circuit 0105 is set so that the program B 0103 can use the memory area.