Container for centrifugation

A container for centrifugation, comprising:

• a central room including a rotation center;
• a through hole opened to the upper part of the central room;
• an outer wall surrounding the central room;
• a ringed dam, not higher than the outer wall, surrounding the central room, wherein the ringed dam includes a surface that faces the central room;
• and
• a peripheral room disposed between the dam and the outer wall,
• wherein plural capillary phenomenon-inducing structures are formed on the surface of the dam facing the central room,
• wherein the container is disc-like,
• wherein the container has a rotation axis, and
• wherein the container is capable of elongating outwardly from the rotation axis in the radial direction, and is capable of rotating around the rotation axis for separating a composite fluid into a low-specific-gravity component and a high-specific-gravity component by centrifugal force.