To detect or determine a heparin affinitive substance in a specimen simply and promptly by letting the heparin affinitive substance react with insoluble carrier particles carrying heparin and an antibody to the heparin affinitive substance.

When a specimen containing a heparin affinitive substance is let to react with insoluble carrier particles such as latex particles or the like carrying heparin and an antibody to the heparin affinitive substance, an aggregate is generated in a suspension state. An absorbance, etc., of the generated aggregate is optically measured directly, whereby the heparin affinitive substance is detected or determined. The insoluble carrier particle used is preferably, one that has an average particle size of 1.6μm or smaller, particularly 0.05-1μm and more particularly 0.05-0.5μm in order to directly measure an immunoagglutination body optically. A material for the insoluble carrier particles is not limited, and for instance, organic polymer substance, inorganic substance, cell membrane, blood corpuscle, microbe, etc., is employed.