Provided are: an oligomer of polyimide copolymer which is an intermediate of a polyimide copolymer having excellent utility and satisfying solvent solubility, storage stability and heat resistance at high levels; a polyimide copolymer obtained therefrom; and their production methods. The oligomer of polyimide copolymer and the polyimide copolymer are obtained by copolymerizing (A) 3,3′,4,4′-biphenyltetracarboxylic dianhydride and/or 3,3′,4,4′-diphenylsulfonetetracarboxylic dianhydride with (B) at least one diamine and/or diisocyanate represented by the following Formulae (1) to (3): (wherein, X represents an amino group or an isocyanate group; R1 to R8 each independently represent a hydrogen atom, an alkyl group having 1 to 4 carbon atoms, an alkenyl group having 2 to 4 carbon atoms or an alkoxy group having 1 to 4 carbon atoms; and at least one of the R1 to R8 is not a hydrogen atom).