Solid polyelectrolyte, assembly of membrane and electrodes, and fuel cell

An object of the present invention is to provide a highly durable solid polymer electrolyte (1) that has a deterioration resistance equal to or higher than that of the fluorine-containing solid polymer electrolytes or a deterioration resistance sufficient for practical purposes, and can be produced at a low cost. According to the present invention, there is provided a solid polymer electrolyte comprising a polyether ether sulfone that is used as an electrolyte and has sulfoalkyl groups bonded to its aromatic rings and represented by the general formula -(CH 2 ) n -SO 3 H.