REACTION VESSEL FOR AUTOMATIC ANALYZER

A reaction vessel capable of measuring a light amount from a reaction liquid without degrading a function of maintaining the reaction vessel at a predetermined temperature is provided. A reaction vessel for an automatic analyzer according one aspect of the invention has a cylindrical shape centered on a first axis, in which an overall length in a first axis direction is longer than an overall length in a second axis direction and an overall length in a third axis direction, 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 includes: an opening part which dispenses a liquid at a portion on one end side in the first axis direction; a first flat surface which has one side extending in the first axis direction and the other side extending in the second axis direction from a portion on the other end side in the first axis direction; and a second flat surface which is substantially parallel to the first flat surface at a portion facing the first flat surface in the third axis direction, in which 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 is less than half the overall length in the first axis direction.