There is provided a serum- or plasma-separating composition capable of maintaining stable blood separation performance over a prolonged period even if subjected to a sterilization step during production or to prolonged storage. The serum- or plasma-separating composition comprises a resin composition having fluidity at normal temperature, a silica fine powder, and an amide-based compound represented by the following formula (1) :wherein R3 is an alkyl group having 1 to 4 carbon atoms and R1 and R2 are independently a hydrogen atom or a linear or branched alkyl group having 1 to 8 carbon atoms.
