To provide a residual cyclic N-vinyllactam based monomer excellent in hue, improved in odor, and to provide a cyclic N-vinyllactam based crosslinked product with a low of soluble components.

The cyclic N-vinyllactam based crosslinked product is produced by polymerizing a cyclic N-vinyllactam based monomer wherein a content of γ-butyrolactone is 30 ppm or less, a content of N-methyl-2-pyrrolidone is 30 ppm or less, and a content of 2-pyrrolidone is 30 ppm or less.