The present invention addresses the problem of providing a novel method for the pretreatment of a biological sample containing lenalidomide enantiomer and thereby establishing a simple and accurate method for the quantitative analysis of lenalidomide enantiomer. In the present invention, the racemization and decomposition of lenalidomide enantiomer in a biological sample can be prevented by the deproteinization under acidic conditions of the biological sample containing lenalidomide enantiomer, and the lenalidomide enantiomer can be simply and accurately quantitatively analyzed by subjecting to HPLC the biological sample that has been pretreated in such a way.