Authors developed a small-sized desktop and low-cost observation device that shows CO_2 concentration changes in real time, aiming that people can experience CO_2 absorption of plants by themselves. The observation device can measure and display CO_2 concentration changes just by putting the small plants inside. Besides, the device enables to accept a certain range of plant size and control environment around the plant for the variety of experiments. Having visitors use the device at the event, "Eco Products 2006", authors found out that they could try some experiments by themselves with the developed device.