A sample processing system comprising: a sample vessel holder including a disk-shaped base section disposed so as to have a central axis extending in a vertical direction, and a sample vessel holding section that is formed coaxially with, and above, the base section, the sample vessel holding section holding a sample vessel in an upright posture; a conveying path (19; 119) that conveys the sample vessel holder placed thereon; and a sample vessel holder holding mechanism (10; 10A; 210; 310) for holding the sample vessel holder including: a holding plate (6; 6A; 306a, 306b) disposed above the base section, a cylindrical columnar space which is arranged to contain the sample vessel holding section, wherein the holding plate (6; 6A; 306a, 306b) is disposed so as to extend along an outer peripheral portion of the cylindrical columnar space, and wherein the holding plate (6; 6A; 306a, 306b) defines an opening through which the sample vessel holding section is able to enter and leave the cylindrical columnar space, and rotational driving means for rotating the holding plate (6; 6A; 306a, 306b) around a central axis of the holding plate (6; 6A; 306a, 306b), characterized in that: the sample vessel holding section has an outside diameter smaller than that of the base section, the central axis of the holding plate corresponds to the vertical central axis of the cylindrical columnar space, and the rotational driving means is arranged to rotate the holding plate (6; 6A; 306a, 306b) from a first position wherein the opening faces toward the upstream side of the conveying path (19, 119) so that the sample vessel holding section can enter the cylindrical columnar space at the upstream side of the conveying path (19, 119) to a second position wherein the opening faces toward a downstream side of the conveying path (19, 119) so that the sample vessel holding section can leave the cylindrical columnar space at a downstream side of the conveying path (19, 119). a sample vessel holder including a disk-shaped base section disposed so as to have a central axis extending in a vertical direction, and a sample vessel holding section that is formed coaxially with, and above, the base section, the sample vessel holding section holding a sample vessel in an upright posture; a conveying path (19; 119) that conveys the sample vessel holder placed thereon; and a sample vessel holder holding mechanism (10; 10A; 210; 310) for holding the sample vessel holder including: a holding plate (6; 6A; 306a, 306b) disposed above the base section, a cylindrical columnar space which is arranged to contain the sample vessel holding section, wherein the holding plate (6; 6A; 306a, 306b) is disposed so as to extend along an outer peripheral portion of the cylindrical columnar space, and wherein the holding plate (6; 6A; 306a, 306b) defines an opening through which the sample vessel holding section is able to enter and leave the cylindrical columnar space, and a holding plate (6; 6A; 306a, 306b) disposed above the base section, a cylindrical columnar space which is arranged to contain the sample vessel holding section, wherein the holding plate (6; 6A; 306a, 306b) is disposed so as to extend along an outer peripheral portion of the cylindrical columnar space, and wherein the holding plate (6; 6A; 306a, 306b) defines an opening through which the sample vessel holding section is able to enter and leave the cylindrical columnar space, and rotational driving means for rotating the holding plate (6; 6A; 306a, 306b) around a central axis of the holding plate (6; 6A; 306a, 306b), the sample vessel holding section has an outside diameter smaller than that of the base section, the central axis of the holding plate corresponds to the vertical central axis of the cylindrical columnar space, and the rotational driving means is arranged to rotate the holding plate (6; 6A; 306a, 306b) from a first position wherein the opening faces toward the upstream side of the conveying path (19, 119) so that the sample vessel holding section can enter the cylindrical columnar space at the upstream side of the conveying path (19, 119) to a second position wherein the opening faces toward a downstream side of the conveying path (19, 119) so that the sample vessel holding section can leave the cylindrical columnar space at a downstream side of the conveying path (19, 119).