Recently, the use of CO 2 concentration monitors as a guide for ventilation is increasing in spaces where many people gather, such as offices. However, 2D map-based visualizations are not enough to understand the progression of indoor air pollution. Therefore, we propose a three-dimensional (3D) visualization of CO 2 concentration using a head-mounted display (HMD). A 3D distribution of CO 2 concentration is automatically calculated using the position coordinates and measured values of fixed and mobile sensors. The results of the preliminary user evaluation suggested that AR visualization may be a more effective way to inform the need for ventilation than conventional methods.