An I/O prescription table (I/O PTBL) is associated with a processing program module that is executed by a conventional sequential execution type processor. Each processing program module fetches the input data according to its associated I/O PTBL and stores the output data in the same table. An execution control program makes reference to the I/O PTBL associated with the processing program module, updates the designated I/O PTBL so as to indicate that the corresponding input data is prepared, and starts the processing program module associated with the I/O PTBL which is furnished with all of the necessary input data.