PURPOSE: To extend the range applicable as a program to be debugged by making start an interactive mode debugger possible from the side of program to debugged.

CONSTITUTION: A debugger calling part 14 is placed in the start position of an object module 12 and calls and starts an interactive debugger 13 when the execution of a program is started. When the conversational debugger 13 is called by the object module 12 in case that the object module 12 is the program to be debugged, the debugger 13 performs initial processings such as reservation of a storage area required for debugger operation, etc. by a debugging environment generating part 17 and starts communication of message output, subcommand input, etc. with a terminal 21 and executes the essential conversational debugging operation.