The authors have proposed a concentration time ratio as a new evaluation index of intellectual productivity, which had been difficult to be quantitatively evaluated, with a concept of concentration on target task, and a measurement tool has been developed based on the index. In addition, a subject experiment was conducted with the tool in which the illumination conditions were changed. As the result, it was found that the index was not affected by learning effect and the difference of intellectual productivity by changing the illumination conditions could be evaluated quantitatively with the index.