Provided are a composite that can be suitably used as an electrolyte in polymer-based solid-state batteries, and various electrochemical devices using the composite. The composite includes a fluorine-containing elastomer and an alkali metal salt as essential components, wherein the fluorine-containing elastomer is an amorphous fluorine-containing elastomer having a glass transition temperature of 25° C. or less, and the composite has a volatile content of 0.1 mass % or less with respect to the entire composite.