Sulfur-containing heterocyclic derivative having beta-secretase-inhibiting activity

The following compound is provided as an agent for treating a disease induced by production, secretion and/or deposition of amyloid ² protein, for example,. a compound represented by the formula (I): wherein ruing A is an optionally substituted carbocyclic group or an optionally substituted heterocyclic group, R 1 is optionally substituted lower alkyl or the like, R 2a and R 2b are each independently hydrogen, optionally substituted lower alkyl or the like, R 3a and R 3c are each independently hydrogen, halogen, hydroxy, optionally substituted lower alkyl or the like, or a pharmaceutically acceptable salt thereof, or a solvate thereof.