The present invention provides, for example, the following compound: wherein ring A is a substituted or unsubstituted carbocycle or a substituted or unsubstituted heterocycle, X 1 -X 2 =X 3 is CR 6 -CR 6 =CR 7 , N-CR 6 =CR 7 , CR 5 -N=CR 7 or CR 5 -CR 6 =N, R 1 is substituted or unsubstituted alkyl, substituted or unsubstituted alkenyl or the like, R 2a and R 2b are each independently hydrogen, substituted or unsubstituted alkyl or the like, R 4a , R 4b , R 5 , R 6 and R 7 are each independently hydrogen, halogen, hydroxy, substituted or unsubstituted alkyl or the like, its pharmaceutically acceptable salt or a solvate thereof having an effect of inhibiting amyloid ² production, especially a BACE1 inhibitory activity, and useful as a medicament for treating diseases induced by production, secretion or deposition of amyloid ² proteins.
