Disclosed is a hydrogel which comprises a polyanionic polysaccharide derivative with a hydrophobic group introduced by covalent bonding and a salt solution with a salt concentration of 50mM-200mM. The manufacturing method for the hydrogel includes: a step wherein a compound including a dilution of a hydrogel with an aggregated structure with an average particle size of 100-200nm, a polyanionic polysaccharide derivative with a hydrophobic introduced thereto, and a salt solution of 50-200mM is prepared and a step wherein the compound is heat-treated. The hydrogel possesses long-term stability, and can be filtered through a porous filter with a pore diameter of 5μm or greater.
