The present invention relates to a serum or plasma separating material including a moisture curing component having a specific gravity of from 1.03 to 1.09, and a blood collection tube including the serum or plasma separating material. The present invention provides a serum or plasma separating material which is allowed to be present in a cured state between serum or plasma and a blood cell-containing component obtained after centrifugal separation when separating the serum or plasma component in a collection tube, exhibits a good storage stability capable of keeping the serum or the like and the cell-containing component in a separated state in the collection tube for a long period of time, is excellent in stability upon freezing or thawing and upon handling of a sample, and can be cured without need of irradiation with ultraviolet ray or the like, as well as a method of separating serum or plasma using the separating material.