The hydrogel includes a first network structure and a second network structure. The second network structure is entwined with the first network structure. The first network structure contains a polymer crosslinked with a first crosslinking agent. The second network structure contains a polymer crosslinked with a second crosslinking agent. 50 mol % or more of the first crosslinking agent does not contain a decomposable bond. 50 mol % or more of the second crosslinking agent does not contain a decomposable bond.