Electrochemical carboxylation is conducted in ionic liquids compressed with carbon dioxide. It is found that the current efficiency increases with temperature and pressure. The diffusion coefficient of the substrate, which is obtained from the electrochemical measurements, also remarkably increases with temperature and pressure. The increase of the current efficiency is mainly explained by improvement in the mass transfer, and the promotion of the electrochemical carboxylation in the ionic liquid is achieved by pressurizing the carbon dioxide moderately.