A 4-membered compound is represented by the following formula (I) is disclosed (in the formula, X1 and X2 each independently represent an oxygen atom, a sulfur atom or a substituted or unsubstituted imino group, Y1 and Y2 each independently represent a single bond, an oxygen atom or a substituted or unsubstituted imino group, B1 and B2 each independently represent an optionally substituted aliphatic, aliphatic carbonyl, aromatic or aromatic carbonyl group having 1-20 carbon atoms, and A1 and A2 each independently represent a group represented by the following formula (II) (Ar1, Ar2 and Ar3 each independently represent a cyclic group having 5-14 carbon atoms, L1 and L2 each independently represent a single bond or a divalent linking group, and p represents an integer of 0-2)). There is also disclosed a birefringence medium containing a 4-membered compound represented by the formula (I) and an optical element comprising the birefringence medium.