To estimate the carbon balance in a wetland ecosystem, gross primary production (GPP) and ecosystem respiration were investigated using the Closed Geosphere Experiment Facility (CGEF). The CGEF is equipped with a Geosphere Module (GM) and a Geosphere Material Circulation (GMC) system. The scale of the GM is 5.8 x 8.7 m² in ground area with an average height of 11.9 m, including the soil of 3.1 m in depth. A wetland ecosystem dominated by a common reed was introduced into the GM. Ecosystem respiration during the night can be easily calculated by measuring the change in air CO₂ concentration and one-hour averaged air temperature and pressure in the GM. Net ecosystem production (NEP) can be easily calculated, the same as in ecosystem respiration. In addition, ecosystem respiration during the day was estimated with air temperature by nonlinear regression. GPP was estimated as the summation of NEP and ecosystem respiration. The CGEF is of great advantage in studying carbon balance in the ecosystem between carbon input as GPP and output as ecosystem respiration, which are difficult to measure precisely in the field.