An electrode structure having a pair of electrode catalyst layers and a polymer electrolyte membrane held between both the electrode catalyst layers is provided. The polymer electrolyte membrane is a sulfonate of a hydrocarbon-based polymer comprising a main chain in which two or more benzene rings are bound together directly or through a divalent group. The membrane includes 5% or more by weight of water coordinated to protons of sulfonic acid groups. The polymer electrolyte membrane may include a fluorine-containing ion conducting polymer such that the ratio of fluorine content in the polymer electrolyte membrane to the fluorine content in the electrode catalyst layers is in the range of from 0.2 to 2.0. The electrode structure constitutes a fuel cell, which generates power when oxidizing gas is supplied to the one side of the electrode structure and reducing gas to the other side.