Sulfur-containing heterocyclic derivative having beta secretase inhibitory activity

A compound represented by the formula (I):

• wherein ring A is an optionally substituted carbocyclic group or an optionally substituted heterocyclic group;
• R1 is optionally substituted lower alkyl, optionally substituted lower alkenyl, optionally substituted lower alkynyl, cyano, an optionally substituted carbocyclic group or an optionally substituted heterocyclic group;
• R2a and R2b are each independently hydrogen, optionally substituted lower alkyl or optionally substituted acyl;
• R3a and R3c are each independently hydrogen, halogen, hydroxy, optionally substituted lower alkyl, optionally substituted lower alkenyl, optionally substituted lower alkynyl, optionally substituted acyl, optionally substituted lower alkoxy, optionally substituted aryl lower alkyl, optionally substituted heteroaryl lower alkyl, optionally substituted aryl lower alkoxy, optionally substituted heteroaryl lower alkoxy, optionally substituted lower alkylthio, carboxy, cyano, optionally substituted lower alkoxycarbonyl, optionally substituted amino, optionally substituted carbamoyl, an optionally substituted carbocyclic group or an optionally substituted heterocyclic group, or R3a and R3c may be taken together to form a ring,
• or its pharmaceutically acceptable salt, or a solvate thereof.