An automatic analyzer equipped with functions to perform an optical measurement and a reaction vessel cleaning, comprising: a control unit for controlling a sequence including the optical measurement and the reaction vessel cleaning; a discharge mechanism including a discharge nozzle for discharging a liquid into a reaction vessel and operative to vertically move up and down the discharge nozzle and to discharge the liquid; and an overflow suction mechanism including an overflow suction nozzle for sucking an overflow amount of the liquid in the reaction vessel and operative to vertically move up and down the overflow suction nozzle and to suck the overflow amount of liquid, wherein in a liquid discharge step which is included in the cleaning process preceding the optical measurement process and which is interposed between a preceding step using a detergent and a succeeding blank value measurement step, the control unit is adapted to provide control so as to carry out the discharge of the liquid from the discharge nozzle and the suction of the overflow amount of liquid through the overflow suction nozzle when the analyzer is in a first state where a lower end of the discharge nozzle is located at a first position in a heightwise lower part of the reaction vessel by a downward movement of the discharge mechanism and a lower end of the overflow suction nozzle is located at a second position in a heightwise upper part of the reaction vessel by a downward movement of the overflow suction mechanism, characterized in that an opening mouth at the lower end of the discharge nozzle has a slant face at a cutting angle of 10 degrees or more, and the slant face is directed to the overflow suction nozzle as seen from the discharge nozzle. a control unit for controlling a sequence including the optical measurement and the reaction vessel cleaning; a discharge mechanism including a discharge nozzle for discharging a liquid into a reaction vessel and operative to vertically move up and down the discharge nozzle and to discharge the liquid; and an overflow suction mechanism including an overflow suction nozzle for sucking an overflow amount of the liquid in the reaction vessel and operative to vertically move up and down the overflow suction nozzle and to suck the overflow amount of liquid, wherein in a liquid discharge step which is included in the cleaning process preceding the optical measurement process and which is interposed between a preceding step using a detergent and a succeeding blank value measurement step, the control unit is adapted to provide control so as to carry out the discharge of the liquid from the discharge nozzle and the suction of the overflow amount of liquid through the overflow suction nozzle when the analyzer is in a first state where a lower end of the discharge nozzle is located at a first position in a heightwise lower part of the reaction vessel by a downward movement of the discharge mechanism and a lower end of the overflow suction nozzle is located at a second position in a heightwise upper part of the reaction vessel by a downward movement of the overflow suction mechanism, characterized in that an opening mouth at the lower end of the discharge nozzle has a slant face at a cutting angle of 10 degrees or more, and the slant face is directed to the overflow suction nozzle as seen from the discharge nozzle.