A pharmaceutical composition for treating Alzheimer's disease containing a compound represented by the general formula (I); wherein ring A is an optionally substituted carbocyclic group or an optionally substituted heterocyclic group; E is lower alkylene, etc.; X is S, O, or NR1; R1 is a hydrogen atom or lower alkyl; R2a, R2b, R3a, R3b, R4a and R4b are each independently a hydrogen atom, halogen, hydroxy, etc.; n and m are each independently an integer of 0 to 3; n+m is an integer of 1 to 3; and R5 is a hydrogen atom, optionally substituted lower alkyl, etc.; its pharmaceutically acceptable salt, or a solvate thereof as an active ingredient.