Solid polymer electrolyte and protonconducting membrane

To provide a sulfonic acid group-containing polymer having improved hot water resistance and radical resistance (durability), a solid polymer electrolyte including the polymer, and a proton-conducting membrane including the electrolyte, the polymer electrolyte includes a sulfonated product of a polymer shown by the following general formula (I): wherein X, Y, and Z are bonded randomly, alternately, or in blocks, y represents an integer of two or more, and each of x and z represents an integer of zero or more, where x+z>2.