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 as a result. When blue light is exposed enough to amount that can improve arousal, is exposed, however, it may cause feeling of glare and disturb their concentration on the contrary. In this study, therefore, an experiment was conducted to reveal the condition where they don’t feel glare and their concentration is not disturbed, when changing the luminance and luminous area of blue light. The Participants participants performed cognitive tasks where blue light source was placed on their work desks under each blue light condition, and then a questionnaire on their subjective glare, fatigue, arousal and concentration was conducted. As a the result, it was found that the reasons why participants felt dazzling are asymmetrical light incidence and large luminous area.