Disclosed is a polymer which is useful for the preparation of an epoxy resin composition or a cured product thereof in film or sheet exhibiting high heat resistance, high thermal conductivity, low thermal expansion, high gas barrier property, and high toughness. The polymer is a thermoplastic aromatic ether polymer comprising a unit represented by the following general formula (1) at a ratio of 10 to 100 mol % and having a weight average molecular weight of 3,000 or more; in formula (1), X is an oxygen atom or a sulfur atom, R1 and R2 each is a hydrogen atom, an alkyl group of 1 to 8 carbon atoms, an aryl group, an alkoxy group, an aralkyl group, or a halogen atom, and n is a number of 1 to 3.