To obtain an inhibitor of neutrophile activity, capable of effectively inhibiting the productivity of an active oxygen by the neutrophile without perishing normal cells, and capable of curing various kinds of diseases such as inflammation caused by the excess release of the active oxygen by including chitosan oxide compound derived from a specific chitosan.

This inhibitor contains a chitosan oxide compound comprising (A) a structural unit of formula I and (B) a structural unit of formula II, regulated so that the molar proportion of the unit A based on the units A+B may be >70 mol%. The conversion of CH2OH groups in the formulas of the units A and B to COOX groups (X is H, Na or K) is 1-100 mol%, and the rate of acetylation of NH2 group in the formula of the unit A is 40-95%. The weight average molecular weight of the chitosan oxide is ≥5,000, and the solubility of the chitosan oxide in water at pH 7 at 25°C is ≥0.1 g/100 g. The chitosan oxide can be applied to the whole preparation because of the water-solubility.