PURPOSE: To attain industrially sufficient glass transition point by expanding the supply of a polymer for non-linear optical medium with a specific epoxy resin.

CONSTITUTION: An epoxy resin expressed by formula is used. In the formula, (x) is 1-40, each of X1 and X2=H or CO-R1-COOH (wherein, R1 is an aliphatic, an alicyclic or an aromatic group), each of Y1 and Y2=D1-G-D2-(each of D1 and D2 is O, S or COO, G is an aliphatic, an alicyclic or aromatic group) or =NR2 (wherein, R2 may be an aryl group substituted by an aliphatic group in some case or an amino-functioned aryl or aromatic aliphatic group in which the amino group is substituted by a glycidyl group), Z is a conjugated π-electron system (E) substituted by an electron acceptor (A) and having a structural formula-E-A.