A method for determining probability that prostate cancer will metastasize, as well as a diagnostic reagent used therefor is disclosed. It was discovered that the percentage that NF-.kappa.B-p65/RelA has the 254th amino acid threonine which is phosphorylated is significantly higher in the prostate cancer cells in the cases where the bone metastasis was observed than in the cases where bone metastasis was not observed. Thus, the method for determining probability that prostate cancer will metastasize comprises measuring human NF-.kappa.B-p65/RelA in which 254th amino acid threonine is phosphorylated, which human NF-.kappa.B-p65/RelA is contained in a prostate tissue separated from human.