Internal modified-mesoporous SiO2 using ionic liquid (IL-MPSi) as one of the additives has been prepared and added to the poly(ethylene oxide) (PEO)-based solid electrolytes for improving the ionic conductivity. In this study, 1-ethyl-3-methylimidazolium tetrafluoroborate (EMImBF4) has been used as an ionic liquid. The conductivity was significantly improved by the addition of IL-MPSi, to be more than 10−5 Scm−1 at room temperature and at the filler contents of 15wt.%, in the solid state. From the combination of dc polarization and ac impedance measurement for lithium cells, it was revealed that the addition of IL-MPSi increased the Li+ transference number (t +), because the EMImBF4 used in this study possesses higher t + (approximately 0.5) than the PEO-salt simple mixture. This means that the MPSi domains can act as “bypass” for transport of carrier ions to the PEO matrix through the inside IL phase.