PURPOSE: To decrease the number of interruption equipment identification numbers, facilitate the extension of an I/O unit, and reduce hardware by allowing a monitor program to pass the right of execution to one of plural task programs according to an interruption from an interruption source and executing the program.

CONSTITUTION: A task program is divided into groups previously and interruption equipment identification numbers are assigned to the respective groups. Then, a common interruption equipment identification number (a) is generated for interruptions from respective I/O units 1W(m). The interruption cntrol program of the monitor program performs control so that the right of execution is passed to respective task program 1W(m) successively for the interruption equipment identification number (a). Each task program when receiving the right of execution checks on the contents of the interruption register of the I/O unit which belongs to the program and decides whether an interruption factor is generated or not. When there is the interruption factor, the task program is executed to perform specific I/O processing, and when no, the right of execution is passed to a monitor.