PURPOSE: To provide an additive to be added to a blood collecting vessel wherein solidification ratio of blood is promoted and none remains in a serum layer and no potential effect such as damage to blood inspection exists, by having a plurality of surface areas, in chemical nature, of the surfaces, wherein one cannot be wetted with a sample blood and the other can be wetted.

CONSTITUTION: The additive put in a blood collection vessel has a plurality of surface areas of different chemical natures. The first surface area can be wetted and solidification cascade is promoted, and the second surface area cannot be wetted, having the nature of absorbing a blood protein, and adsorbs fibrin when it is generated. The most desirable additive is a film-like polystyrene or polypropylene which is treated so that its one surface can be wetted with an oxidizing plasma, and when it is absorbed into a solidified material, a hard and small-volume solidified material is formed, and a lot of serum is obtained from a sample blood. The additive becomes a part of the solidified material when it adsorbs the solidified material, and removed from the serum after deposition by centrifugal operation. Thus, the serum does not contain a foreign substance added for promotion of solidification, and blood inspection is possible as it is.