Provided is a blood separating agent that can form an excellent partition wall in an intermediate layer between clot and serum or between blood cell components and plasma even under low-temperature centrifugation conditions, is less likely to form crevices in the partition wall, is less likely to have any effect on test values, and offers stable performance even after long-term storage. The blood separating agent contains: a polymer for the blood separating agent which is composed of a (meth)acrylic acid ester-based polymer and has a viscosity of 10 to 200 Pa·s at 25° C. and a ratio of viscosity at 15° C. to viscosity at 25° C. of below 4.6; an inorganic powder; and a polyalkylene glycol having a number average molecular weight of 700 or more, wherein the polyalkylene glycol is mixed at a concentration of 5% by weight or less of the total weight of the blood separating agent.