The purpose of the present invention is to provide an adhesion preventing material which is designed so that the fluidity thereof changes with time in such a manner as being in the form of a gel with a high fluidity at administration and, after the administration to a surgical incision/suture site, being in the form of a gel with a low fluidity. A powdery adhesion preventing material to be mixed with an aqueous solvent before using, said adhesion preventing material containing alginic acid and/or a salt thereof and being designed so as to show a viscosity at 37 DEG C of 70 mPas.s or less 5 minutes after mixing with the aqueous solvent, and show a viscosity at 37 DEG C of 120 mPas.s or more 60 minutes after the mixing. When mixed with an aqueous solvent and administered at a clinical site, the adhesion preventing material is maintained in a highly fluid state at the administration and, after the administration to a surgical incision/suture site, the fluidity thereof is lowered to thereby achieve both excellent operability and excellent adhesion preventing effect.本發明之目的是提供一種沾黏防止材,其設計成流動性會經時的變化,使投藥時顯示出流動性高之凝膠狀,且於投藥至業已切開、縫合之部位後顯示出流動性低之凝膠狀。本發明為一種沾黏防止材,其為粉末狀且在使用時是與水性溶劑混合來使用,前述沾黏防止材含有海藻酸及/或其鹽,且是與水性溶劑混合使用以使得在與水性溶劑混合後5分鐘之時間點中,於37℃之黏度為70mPas.s以下,在混合後60分鐘之時間點中,於37℃之黏度為120mPas.s以上,如此沾黏防止材若於臨床現場使用時與水性溶劑混合,則投藥時會保持流動性高之狀態,且於投藥至業已切開、縫合之部位後流動性會降低,而可兼顧優異之操作性與沾黏防止效果。