Previously, we have studied the air-conditioning guidance system (AC-Guide) to optimize the use of AC in the room using the indoor/outdoor discomfort index. Currently, the epidemic of COVID-19 is spreading around the world, and the ventilation of the room becomes critical to avoid the infection, which should be properly guided together. In this paper, we study the ventilation guidance function using the measured CO2 density in AC-Guide. The opening/closing state of the door and window is automatically detected using the web camera, where the image processing procedures are newly implemented. Experiment results in an Okayama University building confirm the effectiveness.