In an execution of a large scale program, even a simple observable behavior may be generated by a wide range of the source code. To comprehend how such a behavior is implemented in the code, a debugger would be helpful. However, when using a debugger, developers often encounter several types of cumbersome tasks and are often confused by the huge and complicated runtime information. To support such a debugger-based comprehension task, we propose an abstraction technique of runtime information, named delta, and present a delta extraction and visualization tool. Basically, a delta is defined for two linked objects in an object-oriented program's execution. It intuitively represents the reason why these objects could be related in the execution, and it can hide the details of how these objects were related. We have conducted experiments on four subject tasks from two real-world systems to evaluate how appropriately an extracted delta can answer the 'why' question and how long the tool can reduce the working time to answer the question. The results show that each delta can successfully answer the question and a tens-of-minutes to one-hour debugger-based task can be reduced by extracting a delta.