Abstract: The present invention provides a phosphonic acid diester compound represented by General Formula wherein R1, R2, R3 and R4 are the same or different, and represent hydrogen, halogen, lower alkyl or lower alkoxy; R5 is phenyl having on the phenyl ring 1-3 substituents selected from the group consisting of lower alkyl, halogen-substituted lower alkyl, lower alkoxy, halogen-substituted lower alkoxy, phenoxy, benzyloxy, hydroxyl, halogen, nitro, lower alkylthio, di(lower alkyl)amino, lower alkanolyamino, pyrrolidinyl and phenyl, or the like, provided that R5 is not mono(lower alkoxy)pheny; and R6 is lower alkyl; and an ACAT-1 inhibitor containing the compound as an active ingredient.