PURPOSE: To obtain a high-speed flow monitor device which has the reduced hard and soft loads, by using a serial IO for multiprocessor system which is incorporated into a 1-chip processor as a monitor output terminal.

CONSTITUTION: A device 1 is controlled by a microprocessor 4 incorporating a serial IO and on the basis of a program in a memory part 3. An output routine of a monitor signal is inserted to a specific area of the above-mentioned program. The processor 4 feeds the monitor signal to a receiving circuit 2 during execution of the program via a serial IO5 and interfaces 6 and 7 to display, print, etc. The codes showing the passing point and branching point of the program are allotted for each label of the program at the area where the output routine of the monitor signal is inserted.