The present invention provides a crystalline lens hardening inhibitor, containing a compound represented by the formula (I):wherein R1-R4 are the same or different and each is a hydrogen atom, a halogen atom, a hydroxy group, a sulfanyl group, a lower alkyl group, a lower acyl group, a lower alkoxy group, a carboxyl group, a carbamoyl group or a carbonylamino acid group, or a salt thereof, and/or a compound represented by the formula (II) :wherein R is a hydrogen atom, or a lower alkyl group optionally substituted by an amino group, a hydroxy group, a sulfanyl group or a carboxyl group, and A is a lower alkylene group, or a salt thereof.
