Appropriate support for debugging contributes to efficient software development. Several previous studies used bio-information such as brain activity to classify the inner-state of programmer without interruption. In this paper, we measure programmer’s brain waves while they comprehend the source code. In the experiment, we analyze difference of time-series brain wave features between success/failure for tasks that source code comprehension and bug judgement. The result of the experiment showed the participants who success source code understanding significantly increased power spectrum of α and β wave with the time passage.