A method for extracting low-molecular-weight proteins/peptides contained in a body fluid sample, particularly, in serum or plasma. The method includes the steps of (a) to (e): (a) adding reagent 1 containing urea and thiourea and reagent 2 containing a reducing agent to the body fluid sample, mixing them, subsequently dropping the mixture into reagent 3 containing 90% or more of an organic solvent, and mixing them; (b) stirring at a low temperature the mixed solution obtained in step (a); (c) centrifuging at a low temperature the stirred solution obtained in step (b) and removing the supernatant; (d) adding reagent 4 containing an organic solvent and an acid to the precipitate obtained in step (c) and mixing them; (e) stirring at a low temperature the mixed solution obtained in step (d); and (f) centrifuging at a low temperature the stirred solution obtained in step (e) and recovering the supernatant.