A polymer electrolyte membrane, wherein the period length L in the membrane surface direction, which period length is defined by formula (1) and is measured by using a small-angle X-ray diffractometer, is less than 52.0 nm: L=λ1/(2 sin(2θi/2))  (1) wherein 2θi represents a scattering angle in the membrane surface direction and λ1 represents the wavelength of X-rays used when the scattering angle in the membrane surface direction is measured.