An automated analyzer comprising: a reactor vessel adapted to accommodate a liquid mixture consisting of a sample and a reagent; a sample dispensation mechanism adapted to dispense the sample to the reactor vessel; a reagent dispensation mechanism adapted to dispense the reagent to the reactor vessel; an analysis unit that includes a plurality of analysis ports each including an optical system formed by a light source for radiating light to the reactor vessel accommodating the liquid mixture and a photodetector unit for receiving the light radiated from the light source; a first movable light shielding mechanism adapted to be moved by a first driving unit along a first direction (117a, 117b) and, when it is positioned above the analysis unit, to shield all the analysis ports from light reflected from the reagent dispensation mechanism; a second movable light shielding mechanism that includes an opening part, wherein the diameter of the opening part is a minimum diameter necessary for allowing the sample and reagent dispensation mechanisms to pass through the second movable light shielding mechanism and to access the analysis ports and wherein the second movable light shielding mechanism is adapted to be moved by a second driving unit along a second direction (118a, 118b) and, when the second movable light shielding mechanism is positioned above the analysis unit, to shield any single one of the analysis ports, except the one analysis port that is below the opening part, from light reflected from the reagent dispensation mechanism; and a control unit adapted to control operations of the sample dispensation mechanism, the reagent dispensation mechanism, the first movable light shielding mechanism, and the second movable light shielding mechanism, wherein, in a state in which all the analysis ports are shielded from the reflected light by the first movable light shielding mechanism, the control unit is adapted to move the second movable light shielding mechanism to place the opening part at a position of a predetermined analysis port, and wherein, in a state in which the opening part of the second movable light shielding mechanism is placed at the position of the predetermined analysis port, the control unit is adapted to move the first movable light shielding mechanism so that the first movable light shielding mechanism does not shield the analysis ports and access to the predetermined analysis port through the opening part of the second movable light shielding mechanism is allowed. a reactor vessel adapted to accommodate a liquid mixture consisting of a sample and a reagent; a sample dispensation mechanism adapted to dispense the sample to the reactor vessel; a reagent dispensation mechanism adapted to dispense the reagent to the reactor vessel; an analysis unit that includes a plurality of analysis ports each including an optical system formed by a light source for radiating light to the reactor vessel accommodating the liquid mixture and a photodetector unit for receiving the light radiated from the light source; a first movable light shielding mechanism adapted to be moved by a first driving unit along a first direction (117a, 117b) and, when it is positioned above the analysis unit, to shield all the analysis ports from light reflected from the reagent dispensation mechanism; a second movable light shielding mechanism that includes an opening part, wherein the diameter of the opening part is a minimum diameter necessary for allowing the sample and reagent dispensation mechanisms to pass through the second movable light shielding mechanism and to access the analysis ports and wherein the second movable light shielding mechanism is adapted to be moved by a second driving unit along a second direction (118a, 118b) and, when the second movable light shielding mechanism is positioned above the analysis unit, to shield any single one of the analysis ports, except the one analysis port that is below the opening part, from light reflected from the reagent dispensation mechanism; and a control unit adapted to control operations of the sample dispensation mechanism, the reagent dispensation mechanism, the first movable light shielding mechanism, and the second movable light shielding mechanism, wherein, in a state in which all the analysis ports are shielded from the reflected light by the first movable light shielding mechanism, the control unit is adapted to move the second movable light shielding mechanism to place the opening part at a position of a predetermined analysis port, and wherein, in a state in which the opening part of the second movable light shielding mechanism is placed at the position of the predetermined analysis port, the control unit is adapted to move the first movable light shielding mechanism so that the first movable light shielding mechanism does not shield the analysis ports and access to the predetermined analysis port through the opening part of the second movable light shielding mechanism is allowed.