Abstract: An oligomer is provided having a polyalkylene glycol main structure, a polymeric double bond group arranged at both ends of the main structure, and a sub-structure arranged between the main structure and each polymeric double bond group. The sub-structures lengthen the main structure, and are composed of a urethane bond and an ethyleneoxy, or a urethane bond, an ethyleneoxy and a propyleneoxy. The introduction of urethane bonds causes crystallization (network formation) with the urethane bonds themselves so that a hydrous gel resulting from polymerizing the oligomer has increased flexibility, strength and erosion resistance. As a result, even though the main structure is lengthened to increase capacity to entrap microorganisms, the strength of the hydrous gel is not reduced. The ethyleneoxy weakens hydrophobicity of urethane regions to improve affinity of the gel for microorganisms. Microorganisms can adhere to and grow on the surface of a hydrous gel produced by polymerizing the oligomer.