A storage portion forming a storage space 10, includes an inclined inner wall portion 20 that is connected to a base portion so that the diameter of the inclined inner wall portion gradually decreases； a concave portion 22 is formed at a part of the inclined inner wall portion； and the concave portion 22 includes a concave portion side surface 22b that is connected to a concave portion bottom surface 22a. The concave portion 22 is formed at a position, where the concave portion crosses an interface S between the specimen centrifuged during rotation and air, in a radial direction with respect to the central axis； and the maximum width of the concave portion 22 in a circumferential direction around the central axis is included in a range of 2 mm to a length of 20% of the whole circumference.
