To obtain a novolak-type phenol-modified copoly (aromatic hydrocarbon/formaldehyde) resin having a short setting time and a high thermal hardness.

This resin is obtained by reacting (A) a copoly (aromatic hydrocarbon/formaldehyde) resin having 2.0-3.0 substituents per one aromatic ring on an average with (B) a phenol in the presence of an acidic catalyst. This resin is also obtained by reacting (A) a copoly(aromatic hydrocarbon/ formaldehyde) resin having 2.0-3.0 substituents per one aromatic ring on an average with (B) a phenol and (C) formaldehyde in the presence of an acidic catalyst. Xylene or mesitylene is preferable as an aromatic hydrocarbon raw material for producing a copoly(aromatic hydrocarbon/formaldehyde) resin.