PURPOSE: To improve the operability of the titled system, by using a pseudo input/output library call instruction received from a compiler to decide the propriety for the execution of a real input/output to call a real input/output library with an operable real input/output and then to perform the pseudo processing with an inoperable real input/output respectively.

CONSTITUTION: A pseudo input/output library call evolving part 12 is added into a compiler 11 and a pseudo input/output library 15 includes a real input/ output ring deciding part 16, a real input/output library call executing part 17 and a pseudo processing part 18. When a debugging option is designated in a source program 10, the part 12 calls and starts the library 15 and the part 16 decides the propriety for execution of the real input/output. When the real input/output is operable, the part 17 calls a real input/output library 20 to perform the actual input/output. While the part 18 carries out the pseudo input/output processing when the real input/output is inoperable.