To provide a processor for preventing unauthorized execution of an authorized program by a user program to secure security and improving the realtime property in the case of authorized branching from the user program to the authorized program.

In a processor provided with a CPU 401 and a flash memory 404 for storing a program, when a branch instruction for changing an operation mode to a different operation mode is executed by the program stored in the flash memory 404, an unauthorized branching detection circuit 409 judges whether a branch permission instruction exists in a branch destination address, and when the branch permission instruction does not exist, an unauthorized branching detection signal is output to prevent unauthorized execution of the authorized program by the user program.