A method for detecting a site which can be modified with an N-linked glycan chain and to which an N-linked glycan chain is actually linked in a glycoprotein; and a method for determining the state of an N-linked glycan chain addition at the site are provided. A glycoprotein having an N-linked glycan chain linked thereto is subjected to an N-linked glycan chain removal treatment with a peptide N-glycanase, subsequently a site capable of being modified with an N-linked glycan chain, in which an Asn residue has been changed to an Asp residue by the action of the peptide N-glycanase, is treated with an endo-type peptidase capable of recognizing an Asp residue to thereby produce peptide fragments, and subsequently the mass of the fragments is detected. In this manner, a site which can be modified with an N-linked glycan chain and to which an N-linked glycan chain is actually linked can be detected. Furthermore, the proportion or state of of the N-linked glycan chain addition at the site can be determined from the intensity of a signal generated upon the detection.