Provided are a polyvinyl alcohol-based resin having an excellent plugging effect and a plugging agent for underground treatment. A polyvinyl alcohol-based resin is disclosed, in which a degree of swelling (%) when the polyvinyl alcohol-based resin is immersed in water at 40° C. for 30 minutes and a solubility (%) when the polyvinyl alcohol-based resin is immersed in water at 65° C. for 5 hours satisfy the degree of swelling/the solubility≥3.0.