The present invention provides a crosslinker used for crosslinking a polymer material having, in the molecule thereof, a multiple bond reactable with a nitrile oxide, the crosslinker including a bifunctional nitrile oxide having an aromatic nitrile oxide derivative structure in which one hydrogen atom of an aromatic ring is substituted with a nitrile oxide group and all hydrogen atoms at an ortho-position of the nitrile oxide group are substituted with a substituent other than a nitrile oxide group, wherein two such aromatic nitrile oxide derivative structures are bonded to either two oxy groups of a di-oxy structure having the two oxy groups or two carbonyl groups of a di-carbonyl structure having the two carbonyl groups.