An evaluation tool for intellectual productivity based on work concentration has been proposed in this study. Base on the fact that the cognitive state of intellectual work is expressed as the transition among (1)work state, (2)short term rest and (3)long term rest, the tool consists of (a)two cognitive tasks and (b)analysis tool to estimate the cognitive states. Here the state (1) and (2) means a concentration state while the state (3) means a non-concentration state. As an experimental result under two illumination conditions, it was found that there was no significant difference of task performance but significance of concentration keeping time by the result of the analysis tool.