An electrolyte membrane (11) includes: a filler (20); and a polymer electrolyte (22). A thickness of the electrolyte membrane (11) is 1 micrometer to 500 micrometer, a moisture content thereof is 10 mass % or more, and a ratio of a swelling ratio in a membrane surface direction (xy) thereof and a swelling ratio in a membrane thickness direction (z) thereof satisfies following Expression 1: where Lambda z is the swelling ratio in the membrane thickness direction (z), and Lambda xy is the swelling ratio in the membrane surface direction (xy). λ ⁢ ⁢ xy λ ⁢ ⁢ z