The purpose of the present invention is to provide a pharmaceutical composition that is useful for the treatment of diseases that are caused by an increase in bone resorption and that does not cause serious side effects even when used in combination with another drug. The present invention relates to: an α-oxoacyl amino-caprolactam derivative that is represented by formula (I) (in the formula, X is —O— or —N(R1)— and R1 represents an alkoxycarbonyl group having 1-10 carbon atoms); and a bone resorption inhibitor containing the α-oxoacyl amino-caprolactam derivative.