Reverse hydroxamic acid derivatives having specific structures represented by the general formula (Ia): (Ia) (wherein A represents hydrogen, etc.; Ar1 represents arylene, etc.; Ar2 represents optionally substituted aryl, heteroaryl, etc.; R1 represents hydrogen, alkyl, etc.; and R2a represents substituted cycloalkyl, heterocycloalkyl, etc.) and the general formula (I): (I) (wherein A, Ar1, ar2, and R1 are the same as defined above and R2 represents hydrogen, R2a, etc.) and salts of the derivatives. They have TNF-α converting enzyme (TACE) inhibitory activity.