We report the gelation mechanism of tetra-armed prepolymer chains in typical aprotic ionic liquid (aIL), i.e., A-B type cross-end coupling reaction of tetra-armed poly(ethylene glycol)s with amine and activated ester terminals (TetraPEGNH(2) and TetraPEGNHS, respectively) in 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([C(2)mIm][TFSA]). In the ion gel system, we focused on the pH (or H+ concentration) dependence of the gelation reaction. We thus applied the protic ionic liquid (pIL), 1-ethylimidazolium TFSA ([C(2)ImH][TFSA]), as a nonvolatile H+ source, and added it into the solvent aIL. It was found that the gelation time of TetraPEG ion gel can be successfully controlled from 1 min to 3 h depending on the concentration of pIL (c(pIL) = 03 mM). This suggests that the acidbase properties of TetraPEGNH2 showing acid-base equilibrium (-NH2+ H+ reversible arrow-NH3+) in the solutions play a key role in the gelation process. The acid dissociation constants, pK(a)s of TetraPEGNH(3)(+) and C(2)ImH(+) (cation of pIL) in aIL were directly determined by potentiometric titration to be 16.4 and 13.7, respectively. This indicates that most of the H+ ions bind to TetraPEGNH(2) and then C(2)ImH(+) exists as neutral C(2)Im. The reaction efficiency of amide bond (cross-linked point) systematically decreased with increasing c(pIL), which was reflected to the mechanical strength of the ion gels. From these results, we discuss the gelation mechanism of TetraPEG in aIL to point out the relationship between polymer network structure and [H+] in the solutions.