An object of the present invention to provide an antiadhesive material designed to have time-varying fluidity as it exhibits a gel form of high fluidity at the time of administration, and exhibits a gel form of low fluidity after it is administered to an incised and sutured site. A powdery antiadhesive material to be mixed with an aqueous solvent upon use containing alginic acid and/or a salt thereof, that is configured to be mixed with the aqueous solvent upon use so that the viscosity at 37° C. is less than or equal to 70 mPas·s at the time of 5 minutes after mixing with the aqueous solvent, and the viscosity at 37° C. is more than or equal to 120 mPas·s at the time of 60 minutes after mixing, will have high fluidity at the time of administration when it is mixed with an aqueous solvent upon use in a clinical scene, and have low fluidity after it is administered to an incised and sutured site, and thus is capable of satisfying both the excellent operability and the antiadhesive effect.
