A reaction vessel with a cylindrical shape centered on a first axis which is used in an automatic analyzer for analyzing at least a part of a specimen using a process of measuring absorbance, an overall length in a first axis direction (z) being longer than an overall length in a second axis direction (x) and an overall length in a third axis direction (y), the second axis being perpendicular to the first axis and the third axis being perpendicular to the first axis and the second axis, the reaction vessel comprising: an opening part which dispenses a liquid at a portion on one end side in the first axis direction (z); a set of facing flat surfaces (54, 55) in the outer shape of the reaction vessel consisting of a first flat surface which is provided at the other end in the first axis direction (z) with one side extending in the first axis direction (z) and the other side extending in the second axis direction (x) and a second flat surface substantially parallel to the first flat surface which is provided at a site opposite to the first flat surface in the third axis direction (y), wherein portions on side surfaces of the first flat surface and the second flat surface are formed to bend in a direction toward an outer side of the reaction vessel, and a length of the first flat surface and the second flat surface in the first axis direction (z) is less than half the overall length in the first axis direction (z), characterized by the reaction vessel further comprising: a protrusion which has a wedge shape whose sharp portion facing a bottom surface side, or a chamfered wedge shape, on an outer side of a cylindrical portion of the reaction vessel for positioning the reaction vessel in the automatic analyzer in the rotation direction along the first axis direction (z) when the reaction vessel is inserted into the automatic analyzer vertically. an opening part which dispenses a liquid at a portion on one end side in the first axis direction (z); a set of facing flat surfaces (54, 55) in the outer shape of the reaction vessel consisting of a first flat surface which is provided at the other end in the first axis direction (z) with one side extending in the first axis direction (z) and the other side extending in the second axis direction (x) and a second flat surface substantially parallel to the first flat surface which is provided at a site opposite to the first flat surface in the third axis direction (y), wherein portions on side surfaces of the first flat surface and the second flat surface are formed to bend in a direction toward an outer side of the reaction vessel, and a length of the first flat surface and the second flat surface in the first axis direction (z) is less than half the overall length in the first axis direction (z), characterized by the reaction vessel further comprising: a protrusion which has a wedge shape whose sharp portion facing a bottom surface side, or a chamfered wedge shape, on an outer side of a cylindrical portion of the reaction vessel for positioning the reaction vessel in the automatic analyzer in the rotation direction along the first axis direction (z) when the reaction vessel is inserted into the automatic analyzer vertically.