There is provided a novel crosslinking agent for use in crosslinked polymer fabrication, specifically a crosslinking agent containing at least one compound selected from glyoxylate and a glyoxylic acid ester derivative represented by the following general formula (1): wherein R1 and R2 each independently represents an alkyl group having 1 to 10 carbon atoms and R3 represents hydrogen or an alkyl group having 1 to 10 carbon atoms.