The inventions provides a method for immunologically determining a keratan sulfate level which method includes bringing an anti-keratan sulfate monoclonal antibody into contact with a biological sample, the anti-keratan sulfate monoclonal antibody exhibiting a relative reaction specificity between keratan sulfate-I and keratan sulfate-II represented by IC50KS-I/KS-II of 0.4 to 5, to thereby provide a signal; and detecting keratan sulfate contained in the biological sample from the signal. On the basis of the method, the invention also provides a joint disease detection method and a method for assessing the effect of a remedy for a joint disease and a candidate substance therefor. Through these methods, a very small amount of keratan sulfate contained in a sample, can be determined. Particularly, these methods can determine, at high-sensitivity and high-specificity, the total keratan sulfate including keratan sulfate-I, which have been difficult to determine through a conventional technique. The methods also enables detect a joint disease and assess the effect of a remedy for a joint disease or a candidate substance therefor.