A compound represented by the following formula (I): wherein R1 represents hydrogen, aryl which may have a substituent, a saturated or unsaturated 5- to 7-membered heterocyclic group which may have a substituent, etc.; R2 represents hydrogen, aryl which may have a substituent, a saturated or unsaturated 5- to 7-membered heterocyclic group which may have a substituent, etc.; R3 represents hydrogen, etc.; Ar represents a divalent group derived from aromatic hydrocarbon, etc.; X represents a single bond, linear or branched alkylene having from 1 to 3 carbon atoms which may have a substituent, etc.; and G represents halogen, a saturated or unsaturated 5- or 6-membered cyclic hydrocarbon group which may have a substituent, a saturated or unsaturated 5- to 7-membered heterocyclic group which may have a substituent, etc., a salt thereof or a solvate thereof; and an agent for inhibiting aggregation and/or deposition of an amyloid protein or an amyloid-like protein, which comprises the compound, a salt thereof or a solvate thereof.