In this study, the authors propose a model which can explain and simulate the change of office work productivity. This model assumes office workers transit a working state and a non-working state with the probabilistic functions based on the variation of brain fatigue. Aiming to discuss the detail of the model, the authors have conducted a subject experiment in which work motivation are controlled. Comparing the results of the experimental results with the result of computer simulation based on the model, it was found that the subjects tend to concentrate the task when the remuneration is provided, and they try to keep their attention to the task and take a long break instead when the task workload is high.