Provided is a solvent-soluble polyimide copolymer which satisfies storage stability and heat resistance at high levels and has excellent utility. The polyimide copolymer is obtained by copolymerizing (A) 3,3′,4,4′-biphenyltetracarboxylic dianhydride with (B) diamine(s) and/or diisocyanate(s) and has no glass transition temperature observed at lower than 500° C. The component (B) is preferably 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).