We report that water-in-ionic liquid microemulsions (MEs) are stably formed in an organic solvent-free system, i.e., a mixture of aprotic (aIL) and protic (pIL) imidazolium-based ionic liquids (ILs) containing anionic surfactant, dioctyl sulfosuccinate sodium salt (AOT). Structural investigations using dynamic light, small-angle X-ray, and small-angle neutron scatterings were performed for MEs formed in mixtures of aprotic 1-octyl-3-methylimidazolium ([C8mIm+]) and protic 1-alkylimidazolium ([CnImH+], n = 4 or 8) IL with a common anion, bis(trifluoromethanesulfonyl)amide ([TFSA-]). It was found that the ME structure strongly depends on the mixing composition of the aIL/pIL in the medium. The ME size appreciably increases with increasing pIL content in both [C8mIm+][TFSA-]/[C8ImH+][TFSA-] and [C8mIm+][TFSA-]/[C4ImH+][TFSA-] mixtures. The size is larger for the n = 8 system than that for the n = 4 system. These results indicate that the shell part of MEs is composed of both AOT and pIL cation and the ME size can be tuned by pIL content in the aIL/pIL mixtures.