To provide a modifier which can be compounded with an epoxy resin composition comprising an epoxy resin and a hardening agent to improve flame retardancy and adhesiveness and lower elastic modulus, when heated, to provide its production method, and to provide an epoxy resin composition compounded with the same.

This modifier for epoxy resins comprises an aromatic hydrocarbon resin having a softening point of 50 to 150°C and obtained by condensing a bifunctional aromatic compound such as p-xylylene glycol or 4,4'-bischloromethylbiphenyl with an aromatic hydrocarbon such as naphthalene in the presence of an acid catalyst, and an epoxy resin composition containing the same.