It is known that blue light exposure to the eyes improves our arousal level. It is expected that exposure of office workers to blue light can maintain their concentration on their intellectual work and it may improve efficiency of their work. When blue light is exposed enough to improve arousal, however, it may cause feeling of dazzling and disturb their concentration on the contrary. In previous studies that were hypothesized and evaluated that the awakening improvement effect by blue light exposure leads to an increase in concentration on intellectual work, the intellectual concentration improvement effect was not seen because of glare. Therefore, we conducted an experiment to evaluate what kind of light conditions would interfere with intellectual work due to glare when changing the luminance and luminous area of blue light source. In this experiment, the participants performed cognitive tasks where blue light source was placed on their desks under one of four blue light conditions. As the result, it was found that the reasons why participants felt dazzling are not only asymmetrical intense light exposure but also large luminous area.