Effects of the alkyl-chain length of the imidazolium cation on the mixing state of imidazolium-based ionic liquids, 1-alkyl-3-methylimidazolium (C_nmim⁺, the alkyl-chain lengths n of 4, 6, 8, 10, and 12) bis(trifluoromethanesulfonyl)amide (TFSA⁻), and methanol were investigated using small-angle neutron scattering (SANS), attenuated total reflectance infrared (ATR-IR), and NMR techniques. SANS measurements revealed that C_nmim⁺TFSA⁻ is heterogeneously mixed with methanol in the methanol mole fraction range of 0.8 ≤ x_CD3OD ≤ 0.995. The heterogeneity of the C_nmim⁺TFSA⁻–methanol solutions, except for C₄mim⁺TFSA⁻, is most enhanced at x_CD3OD ≈ 0.97 over the entire mole fraction range. Thus, the mole fraction at the maximum heterogeneity of the solutions is independent of the alkyl-chain length. In contrast, the magnitude of the maximum heterogeneity of the solutions is larger in the order of the alkyl-chain length from n = 4 to 12. ATR-IR and NMR measurements showed that methanol molecules gradually form hydrogen bonds among them in the solutions with increasing x_CH3OH. In particular, the hydrogen-bonds among methanol molecules are conspicuously evolved in the solutions above x_CH3OH ≈ 0.8. The increase in the concentration of the hydrogen-bonded methanol with increasing x_CH3OH does not significantly depend on the alkyl-chain length. According to these results, we concluded that the heterogeneity of C_nmim⁺TFSA⁻–methanol solutions arises from polar domains composed of the imidazolium rings, TFSA⁻, and methanol clusters and nonpolar domains formed by interaction among the alkyl chains of the imidazolium cations.