We propose a model for analyzing the reading strategies in software debugging. The model provides quantitative and objective visions to a human's debugging activity, and provides the framework for clarifying good- and/or bad-strategies for program reading. We have conducted a case study to observe the debugging processes under a controlled environment. The observation includes: both novice debugger and expert debugger could correctly locate an area that seems to have a bug, however, only the expert subject could quickly narrow down that area, reading the faulty (or most suspicious) module only will not generally lead to a shorter debugging time, and the most well-performed subjects read the module that seems to be a key to find a fault. This case study suggested that explicit and quantitative evaluation of the debugging process becomes possible by using the proposed model.