Abstract: It is intended to provide: a peptide which shows an increase in the expression amount in the serum of a cancer patient with the passage of time before the onset of bone metastasis and after the onset of bone metastasis, is adsorbed by a weak cation exchanger at pH 6, and has an apparent molecular weight determined by mass spectrometry of about 5,800; a peptide fragment comprising the amino acid sequence corresponding to from 10- to 62-positions of the amino acid sequence of matured azurocidin; and a method of diagnosing bone metastasis in a cancer patient characterized by comprising detecting the expression of one of the above peptides in a biological sample originating in the cancer patient. According to this method, bone metastasis can be found and treated at the early stage.