PURPOSE: To obtain a phenolic resin oligomer in a high yield while suppressing the production of byproducts having a high or a low molecular weight, useful as a curing agent, etc., for an epoxy resin by reacting in a specific solvent.

CONSTITUTION: This phenolic resin oligomer expressed by formula III is obtained by reacting phenols of formula I (x is 1, 2, 3; R1 is a 1-4C alkyl; y is 0, 1, 2, 3) with diformylphenols of formula II (R2 is H, a 1-10C alkyl, etc.; z is 0, 1, 2, 3) in the presence of an acid catalyst (e.g. formic acid) in a polar aprotic solvent (e.g. N,N-dimethylformamide). Further, the above polar aprotic solvent has ≥2.5D dipole moment and ≥15 dielectric constant.