To realize a debug write function without re-compiling a program in debugging the program by replacing the instruction of a position at which the display of an executed state is desired with an exception generating code, and executing a program to be debugged.

The instruction of a position to be called on a program 11 to be debugged is replaced with an exception generating code 14, when the execution of the program 11 reaches the position, an exception is generated to break, and an executed state displaying function 21 is called in an exception processing by an exception processing means 16. The called executed state displaying function 21 receives an object 15 to be displayed such as the value of a variable on the program 11 and a display content preserved in a display request preserving area 18 from a display request managing function 20, and automatically displays the executed state of the program 11 on a displaying means 13. Afterwards, the instruction replaced with the exception generating code 14 is returned to the original instruction code, and the program 11 is re-executed in returning from the exception.