Here, we investigate and discuss the effect of accuracy of imitation for improvement of skills on brain activity. In order to improve the skills, learners combine and accumulate information of the skills through practice. Thus, we used near-infrared spectroscopy (NIRS) to investigate brain activity during the process of improvement. Evaluation of the level of knowledge acquisition with monitoring of brain activity can be an indicator of the learner's degree of skill progression. Therefore, our final goal is constructing a new learning model based on brain activity monitoring and improving learning efficiency. We experimented on the assembly operation by imitation learning that assumed work in the manufacturing industries from a previous example. As a result, we showed the possibility of brain activity shift with improvement of the skill. In this article, we targeted task accuracy and investigated whether the brain activity shift is caused by a progress in the task accuracy, act of practice, or some other factor. As a result, we showed a possibility that the trend shift in the right and left dorsolateral prefrontal area and frontal pole was not caused by the simple task accuracy improvement but by the action of practice, which helped subjects store the information.