A process for preparing a thermosetting imide type prepolymer which comprises heating a bis-imide (A) and a diamine (B) in the presence of an organic solvent with a boiling point of 70.degree. to 170.degree. C. and heating the reaction mixture together with an epoxy compound (C). The imide type prepolymer prepared according to the process of the present invention has rapid curability and excellent solubility in low boiling point solvents such as acetone, methyl ethyl ketone and methyl cellosolve, which are available at low cost. Accordingly, the prepolymer is advantageously used especially for the production of laminated boards having good heat resisting properties, low moisture absorption and good adhesion to various substrates such as a copper foil.