The NASICON-type doped LiTi(PO4)(3) has high lithium -ion conductivity and is stable in aqueous solution with a high lithium-ion content, but is unstable in contact with lithium metal. Therefore, these electrolytes should be protected by a lithium-stable lithium-ion conductor for a cell with a lithium metal electrode. Here we propose a composite solid electrolyte of a high lithium-ion conducting solid electrolyte film, Li1.4Al0.4Ge0.2Ti1.4(PO4)(3) (LAGTP), coated with a thin polyethylene oxide (PEO) based polymer electrolyte layer. LAGTP coated with PEO containing an ionic liquid and an oxide filler of BaTiO3 (or SiO2) was stable in contact with lithium metal and the interface between the lithium metal and composite electrolyte exhibited low polarization during the lithium deposition and stripping processes.