The present invention provides a method of obtaining an index value used for pathological tissue diagnosis of prostate cancer, which method has low invasiveness and can be performed at a low cost. The method according to the present invention is a method of estimating a Gleason score that represents the malignancy of prostate cancer, which method includes: measuring the content of a prostate-specific antigen (PSA) having an N -acetylgalactosamine residue at a non-reducing terminal of a sugar chain in a sample; and estimating that the Gleason score is 7 or higher when the thus measured value is larger than a threshold value, or estimating that the Gleason score is 6 or lower when the measured value is smaller than a threshold value. The PSA is preferably quantified by a method including the step of binding a molecule having an affinity for ²-N -acetylgalactosamine residue, such as Wisteria floribunda lectin (WFA), soybean agglutinin (SBA), Vicia Villosa lectin (VVL) or an anti- ²-N- acetylgalactosamine antibody, to the PSA.
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