Provided is a method for diagnosing cancer through a difference with a control group in view of the ratio of a deglycosylated peptide fragment and a non-glycosylated peptide fragment in a protein including an N-linked glycosylation motif. Further provided is a method for diagnosing cancer through the detection of the glycosylation ratio in the protein according to the subject matter enables the diagnosis of cancer with high specificity and sensitivity using at least one existing marker, and can be useful in discovering new markers for diagnosing cancer.