The invention provides a serum or plasma separating composition which can compensate for the lack of initial thixotropy even at a reduced inorganic powder concentration and in which crevices or the like are less likely to be created in a partition wall formed of the separating composition after being centrifuged. The serum or plasma separating composition contains a liquid resin component having a partition wall-forming capability, a hydrophilic inorganic powder, a hydrophobic inorganic powder, and an organic compound serving as a thixotropy enhancer, wherein the organic compound is a polyalkylene glycol and/or a derivative thereof and is made of at least one type of polymer.