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 37C of 70 mPass or less 5 minutes after mixing with the aqueous solvent and show a viscosity at 37C of 120 mPass 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.
