Provided is 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 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 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 prostate-specific antigen is preferably quantified by a method including the step of binding a molecule having an affinity for β-N-acetylgalactosamine residue, such as Wisteria floribunda lectin, soybean agglutinin, Vicia Villosa lectin or an anti-β-N-acetylgalactosamine antibody, to the prostate-specific antigen.
