PURPOSE: To improve the producing efficiency of a debug sentence by adding a deciding part for conditions to the debug sentence contained in a source program, storing the format of the debug sentence and its converted format obtained at execution of the debug sentence into a conversion table, and producing an executing sentence based on the result of comparison between the read source program and the conversion table.

CONSTITUTION: A debug sentence which delivers the information for test is put into a source program and a condition deciding part S1 is added to the debug sentence. Then the execution of the debug sentence is controlled based on the due conditions when the source program is executed. When a source program executing sentence is produced, the format of the debug sentence and its converted format obtained at the execution of the debug sentence are stored into the conversion tables 22 and 42. Then the source program is read every line and compared with both tables 22 and 42. In case no debug sentence is obtained from the results of said comparison, an original program is compiled. While the debug sentence if obtained is processed according to a control variable or the executing sentence.