The present invention provides a therapeutic agent, a treatment method and an inspection method for diseases caused by activation of neutrophils. More specifically, the invention relates to a neutrophil activation regulator which comprising a histidine-rich glycoprotein (HRG) as an active ingredient, and provides a therapeutic agent for diseases caused by neutrophil activation comprising the neutrophil activation regulator, a treatment method for diseases caused by neutrophil activation, and further an inspection method for diseases caused by neutrophil activation. The present invention is based on the neutrophil activation regulator including the HRG as an active ingredient. The present invention extends to the depressant agent for neutrophil-vascular endothelial cell interaction including the HRG of the present invention as an active ingredient, the treatment method for diseases caused by neutrophil activation and/or inflammatory diseases accompanied by neutrophil activation. The present invention also extends to the inspection method for inflammatory diseases accompanied by neutrophil activation by measuring the blood HRG level.