An aqueous solution of monoethanol amine (MEA) has been utilized in an industrial process of CO2 absorption. The chemical reaction between CO2 and MEA, which is employed in the process, consists of two steps. After the formation of the MEA-CO2 complex ("capture"), a proton transfers from the complex to give a final product. In the present study, the overall mechanism of the reaction is discussed, especially focusing on the proton transfer step. Using RISM-SCF-SEDD, a hybrid method of electronic structure theory and statistical mechanics for molecular liquid, we clarified that the role of MEA as a base is crucial in the proton transfer step.