The present invention is related to a polymer comprising terminal sulfonic acid groups, which has a high ionic conductivity and good structural stability, does not decompose even under low humidity conditions, and is inexpensive to produce. Furthermore, the present invention is related to a polymer electrolyte and a fuel cell using the same. The polymer of the present invention has a substituent comprising a terminal sulfonic acid group comprising Formula (1) at a side chain: ##STR00001## where n may be a number in the range of about 1 to about 5.