The present invention provides for debugging a target program with a computer comprising a storage unit and a processing unit. In one aspect, this comprises: storing, in the storage unit, reliability information including the reliability of individual indices of classification items of the program; reading, by the processing unit, the reliability information of the target program from the storage unit and determining a reliable code area and an unreliable code area by using the reliability of the individual indices in the reliability information; and executing debugging of the debug target program only for the unreliable code area while omitting debugging of the reliable code area. A graphical user interface to support debugging of only unreliable code areas is also disclosed.