Silver-ion conducting organic-inorganic composites were prepared. An organic thermo-elastic polymer (SEPS: styrene-ethlene-proplylene-styrene co-polymer) and an inorganic silver ion conductor RbAg₄I₅ which was prepared by mechanical milling processes were used as starting materials for the preparation of the composite materials. The mixture of the starting materials were heated at 90-100°C under a pressure of about 4×10⁶ Pa for 1 h to prepare the composite sheets. The obtained composite sheet containing 25 vol% of SEPS showed ion conductivity 5.3×100 Sm^-1 at room temperature. The Ag⁺ ion transport number of the obtained sheets was almost unity.