Recent observations have shown that carbon in the gas around beta Pictoris is more than 100 times over- abundant with respect to the solar abundance. Although it is thought that such an overabundance in carbon is crucial to retain the metal elements in the disk, its origin is however unclear. In this paper, we establish a simple analytical model to study gas the removal process and thus calculate the abundance of various elements in the gas disk around beta Pictoris. The gas removal rate is controlled by the inward flow from viscous accretion and the outward radiation-induced drift. If the disk viscosity (using classical alpha-disk model) is low, radiation drift dominates the gas loss, and carbon can become highly overabundant. In order to produce the observed overabundance of carbon, a low viscosity of alpha < 10^-3 and a gas production with solar abundance are preferred.