PURPOSE: To shorten the time for program development and to improve the reliability of each software by performing the operation between one of two program memories, and a CPU by a switching control part in accordance with a program select signal.

CONSTITUTION: When a switching instruction is sent from a subscriber terminal 1 to a CPU 3 through a signal level converting part 2, the CPU 3 sends a program select signal SEL to a switching control part 4. When receiving this signal SEL, the switching control part 4 gives an operating signal to prescribed one of two program memories 5 and 6 to designate it. Data processing is performed between only the designated program memory and the CPU 3 through a data bus and an address bus. Thus, a desired program is automatically executed from the subscriber terminal. When a power-on reset signal accompanied with power-on is given to the switching control part 4, one preliminarily determined program memory can be operated.