The present invention is related to a polyimide, which has high ionic conductivity and good structural stability, does not decompose even under low humidity, and is inexpensive, and a polymer electrolyte and a fuel cell using the same. In particular, the polyimide of the present invention has a repeating unit of Formula (1) below and also includes a reactive blocking group derived from a monoamine compound or monoanhydride compound having at least one carbon-carbon double bond at a terminal or at both terminals: ##STR00001##