The reaction rate for the polycondensation in the Tetra-PEG gel system, i.e., A-B type coupling reaction between mutually reactive two four-arm polymers, has been studied by ATR-IR spectroscopy. It was found that (1) the polycondensation kinetics of Tetra-PEG gel can be simply treated as a chemical reaction between mutually reactive end-groups in solution, (2) the reaction undergoes as a simple second-order reaction from beginning to end regardless of gelation threshold, and (3) the gelation mechanism was predicted from the thermodynamic enthalpy and entropy at the transition state estimated by temperature dependence of rate constants. The reson of smooth second-order kinetics is suspected to be that the mean-field approximation can be applied to the reactivity of terminal groups on Tetra-PEGs; i.e., the reactivity of terminal groups on Tetra-PEGs is not affected by the steric hindrance, substitution effet, and gelation threshold.