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 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 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.
