[Constitution] A tricyclic heterocyclic compound represented by general formula (I) wherein R¹ and R² represent each hydrogen, lower alkyl, lower alkoxy, halogen or halogenated lower alkyl; R³ represents hydrogen or lower alkyl; R⁴ represents substituted phenyl or naphthyl; R⁵ represents hydrogen or lower alkyl; A represents lower alkylene; B represents -O- or -S-; and n represents 0 or 1. [Effect] It is excellent in the inhibition of acyl-CoA-cholesterol acyltransferase (ACAT) and is useful for treating or preventing arteriosclerosis.