As a quantitative index to evaluate intellectual productivity, a concentration time ratio (CTR) has been proposed in this study, which is the time ratio between concentration state and non-concentration state on intellectual work. The CTR can be calculated from the solving time distribution when cognitive tasks with unified difficulty are given repeatedly. In addition, an appropriate cognitive task to measure the CTR has been designed and an experiment was conducted with 24 male subjects for three days where the CTRs under two illumination conditions had been compared. As the result, it was found that the CTR was not affected by learning effect and it has a possibility to evaluate the change of intellectual productivity affected by the change of room environmental conditions.