The present invention provides a compound which has an effect of inhibiting amyloid β production, especially an effect of inhibiting BACE1, and which is useful as a therapeutic or prophylactic agent for diseases induced by production, secretion and/or deposition of amyloid β proteins.A compound of the formula (I): wherein X is —C(R3a)(R3b)—, —C(R3a)(R3b)—C(R3c)(R3d)— or —C(R3a)═C(R3c)—, R1 is substituted or unsubstituted alkyl or the like, R2a, R2b, R3a, R3b, R3c and R3d are each independently hydrogen, halogen or the like, R4 is hydrogen or halogen, Ring B is substituted or unsubstituted carbocycle or a substituted or unsubstituted heterocycle, or a pharmaceutically acceptable salt thereof.