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
