PURPOSE: To make it unnecessary to make up a control program in accordance with constitution of a connected input/output device, by providing a memory part for storing the control program for driving each input/output controlling part, on each input/output controlling part.

CONSTITUTION: A magnetic disk device controlling 3 is drawn out from a mother board connecting part 30, contains an address bus line 31 connected to a memory part 40 or an address deciding circuit 44, and a memory part controlling line 33 to be led to the memory part 40, and also a magnetic disk controlling mechanism 42 is connected to the magnetic disk device through a magnetic disk connecting part 43. When the upper rank part of the address bus 31 has become a prescribed value, an address deciding circuit 44 makes the memory part 40 enable through a control line 35, so that the memory part 40 using the memory part controlling line 33 and a data bus 32 can be accessed. Also, when the upper rank part of the address bus 31 is other value, the memory part 40 is not accessed.