The absorption process of a single gas-bubble into a liquid was investigated both experimentally and numerically. We estimated numerically the effect of natural convection due to the concentration difference between a pure liquid and the gas-absorbed liquid, so-called ’density-induced natural convection’, by directly solving the gas diffusion equation in liquid, and the vorticity transport and Poisson equations for the stream function. We estimated experimentally the diffusion coefficient of gas in a liquid by measuring the solution times of a single gas-bubble under a plate. Both the experimental and numerical results reveal that when the initial bubble radius is large, the solution time is affected by the density-induced natural convection. Moreover, in the region where the effect of the density-induced natural convection is minimal, the absorption process can be expressed by two nondimensional parameters, namely, the ratio of gas density inside and outside the bubble and a parameter representing the strength of natural convection. These parameters help clarify the conditions where the solution process is not affected by natural convection.