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 37oC of 70 mPas.s or less 5 minutes after mixing with the aqueous solvent, and show a viscosity at 37oC 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.
