The present invention relates to a norbornene-based crosslinked polymer containing at least one member selected from the group consisting of dicyclopentadiene-based monomer units, tetracyclododecene-based monomer units, and tricyclopentadiene-based monomer units in an amount of 50% by mass or more, wherein the norbornene-based crosslinked polymer has a glass transition temperature of 240° C. or higher. Further, the present invention relates to a method for producing a norbornene-based crosslinked polymer as defined above, including step (1): heating a blend containing at least one member of the above monomer components, and a metathesis polymerization catalyst to a temperature lower than a deactivation temperature of the metathesis polymerization catalyst to carry out a primary curing; and step (2): heating a cured product obtained in the step (1) to a temperature equal to or higher than the deactivation temperature of the above metathesis polymerization catalyst to carry out a secondary curing.