A copolymer having a polystyrene reduced number average molecular weight of 103-108, and comprising repeating units represented by formulas (1), (2), and (3), 1 wherein, R1 and R2 independently show a hydrogen atom, alkyl group, etc.; R3 and R4 independently show alkyl group, etc.; and m and n independently show an integer of 0-3, 2 wherein, R5 and R6 independently show a hydrogen atom, alkyl group, etc.; either R5 or R6 is different from either R1 or R2; R7 and R8 independently show an alkyl group, etc.; and k and l are independently an integer of 0-3, nullAr1nullnullnull(3) wherein, Ar1 shows an arylene group, etc. The copolymer has an excellent solubility in an organic solvent, and a high glass transition temperature in good balance, a polymer LED of high performance, such as low-voltage driving and high luminance, is provided.