PURPOSE: To prevent difficulty at operation of a main program despite of a slow answer of an I/O device by controlling an I/O interface via various I/O interface control parts and performing other processes through a common control part.

CONSTITUTION: A common program control part 1 decides the input/output conditions, etc. of a central controller and an I/O device and produces a head address of a microprogram stored in a common memory 3 to send it to a common program control sequencer 2. An I/O program selection part 7 decides the input/ output conditions of various I/O devices and produces the head addresses of the microprograms stored in the I/Oa and I/Oj control program memories 9 and 11, etc. An I/O control interruption signal generating part 12 outputs an interruption signal to a common control program and uses properly the program 3 and an I/O control program for execution of transfer of data to the I/O device.