NOVELTY - An oligomer solid acid comprises a main chain having a degree of polymerization of 10-70; and a side chain having ionically conductive terminal group bonded to a repeating unit of the main chain.    USE - For polymer electrolyte membrane of fuel cell (claimed).    ADVANTAGE - Since the number of ionically conductive terminal groups in the polymer electrolyte membrane is minimized and the polymer matrix in which swelling is suppressed is used, methanol crossover and difficulties of outflow due to a large volume are minimized, and a macromolecule of the oligomer solid acid having the ionically conductive terminal groups on the surface thereof is uniformly distributed. Accordingly, ionic conductivity is high and thus, the polymer electrolyte membrane shows good ionic conductivity even in low humidity conditions. The oligomer solid acid is not separated easily from the polymer electrolyte membrane.    DETAILED DESCRIPTION - An oligomer solid acid comprises a main chain having a degree of polymerization of 10-70; and a side chain having the structure of formula -E1-...-Ei-...-En (1) bonded to a repeating unit of the main chain.    Ei=included in E1 through En-1, organic groups of formulae (2)-(6);    En=-SO3H, -COOH, -OH, or -OPO(OH)2; and    n=2-4 and indicates the generation of branching unit.    The number of Ei+1 of the (i+ 1)th generation bonded with Ei of the ith generation is the same as the number of available bonds existing in Ei. INDEPENDENT CLAIMS are included for:    (1) a polymer electrolyte membrane comprising at least one polymer matrix having an end group consisting of En at the terminal of a side chain, and the oligomer solid acid uniformly distributed through the polymer matrixes;    (2) a membrane electrode assembly (MEA) (21) comprising the polymer electrolyte membrane; a cathode on a first side of the polymer electrolyte membrane and having a catalyst layer and a diffusion layer; and an anode on a second side of the polymer electrolyte membrane and having a catalyst layer and a diffusion layer; and    (3) a fuel cell (100).    DESCRIPTION OF DRAWING(S) - The figure shows a fuel cell.    Fuel cell stack (7)    MEA (21)    Separators (23, 25)    Fuel cell (100)