The present invention provides a CETP activity inhibitor comprising as an active ingredient a compound represented by the formula (I): wherein R represents a straight chain or branched alkyl group; a straight chain or branched alkenyl group; a lower haloalkyl group; a substituted or unsubstituted cycloalkyl group; a substituted or unsubstituted cycloalkenyl group; a substituted or unsubstituted cycloalkylalkyl group; a substituted or unsubstituted aryl group, or a substituted or unsubstituted heterocyclic group, X1, X2, X3, and X4 may be the same or different and each represents a hydrogen atom, a halogen atom, a lower alkyl group, a lower haloalkyl group; a lower alkoxy group; a cyano group; a nitro group; an acyl group; or an aryl group, Y represents —CO— or —SO2—, and Z represents a hydrogen atom or a mercapto-protecting group, or a prodrug compound, a pharmaceutically acceptable salt, or hydrate or solvate thereof. The compounds represented by the formula (I) can increase HDL and at the same time decrease LDL through selective inhibition of CETP activity and, therefore, is expected to be useful as a new type of a preventive or therapeutic agent for atherosclerosis or hyperlipidemia.