This invention relates to a method for preventing and/or treating bone diseases which comprises administering a potentiator of TGF-beta activity such as the compound of the formula [I] or pharmaceutically acceptable salts thereof to human being or animals. wherein R1 is hydrogen, etc., R2 is optionally substituted ar(lower)alkyl or optionally substituted heterocyclic(lower)alkyl, etc., R3 is hydrogen, optionally substituted hydroxy, etc., R4 is hydrogen or lower alkyl, and X is CH or N.