A bifunctional phenylene ether oligomer of the formula (1), obtained by oxidation polymerization of a bivalent phenol of the formula (2) and a monovalent phenol of the formula (3), ##STR00001## wherein --X-- is represented by the formula (2'), ##STR00002## and Y--O-- is represented by the formula (3), ##STR00003## R2, R3, R4, R8, R9, R10 and R11 in the formula (2') and the formula (3') being required not to be a hydrogen atom, and its use.