The following compound is provided as an agent for treating a disease induced by production, secretion and/or deposition of amyloid β protein, for example, 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 or the like, R2a and R2b are each independently hydrogen, optionally substituted lower alkyl or the like, R3a and R3c are each independently hydrogen, halogen, hydroxy, optionally substituted lower alkyl or the like, or a pharmaceutically acceptable salt thereof, or a solvate thereof.