The human frequency discrimination threshold has been found in a previous experiment. However I do not think that the sound distinguished is its numerical value obtained from a previous study. In this experiment, I use A4 (=440Hz) in 12 equal temperaments. Using the fuzzy set theory I express the vagueness of frequency discrimination in range between 427.47Hz and 452.89Hz (±50 cent from A4).From fuzzy inference result, I found that frequency discrimination is vagueness and the difference between "high frequency discrimination threshold" and "low frequency discrimination threshold" is not so large. I think that the frequency discrimination ability may rise for the subjects having musical experience because the shape changes in distributions.