An automatic analysis apparatus comprising: a reaction container moving mechanism configured to have a plurality of reaction containers for containing a specimen and a reagent arranged in the reaction container moving mechanism, to have a reaction vessel where the reaction containers are immersed in water, and to move the reaction containers; a light source lamp configured to irradiate the reaction container arranged in the reaction vessel of the reaction container moving mechanism with light; a photometer configured to measure the light emitted from the light source lamp and passed through the reaction container; a discharge mechanism configured to discharge water from the reaction vessel; a water supply mechanism (48, 54) configured to supply water into the reaction vessel; a reaction vessel water level detector configured to include a first water level detector for detecting that a water level of the water in the reaction vessel is a full water level; and a control unit configured to control operations of the reaction container moving mechanism, the light source lamp, and the photometer, to analyze the specimen in the reaction container, and to control operations of the discharge mechanism and the water supply mechanism based on the water level of the reaction vessel water detected by the reaction vessel water level detector, characterized in that the reaction vessel water level detector includes a second water level detector for sensing that the water level of the water in the reaction vessel is a measurement limit water level which is lower than the full water level and higher than a position of an optical axis of the light emitted from the light source lamp; wherein the control unit is configured to perform a reaction vessel water replacing operation in which the water in the reaction vessel is drained to the measurement limit water level by controlling the discharging mechanism when the water level of the water in the reaction vessel is the full water level, and the water is supplied to the full water level by controlling the water supply mechanism when the water level of the water in the reaction vessel reaches the measurement limit water level. a reaction container moving mechanism configured to have a plurality of reaction containers for containing a specimen and a reagent arranged in the reaction container moving mechanism, to have a reaction vessel where the reaction containers are immersed in water, and to move the reaction containers; a light source lamp configured to irradiate the reaction container arranged in the reaction vessel of the reaction container moving mechanism with light; a photometer configured to measure the light emitted from the light source lamp and passed through the reaction container; a discharge mechanism configured to discharge water from the reaction vessel; a water supply mechanism (48, 54) configured to supply water into the reaction vessel; a reaction vessel water level detector configured to include a first water level detector for detecting that a water level of the water in the reaction vessel is a full water level; and a control unit configured to control operations of the reaction container moving mechanism, the light source lamp, and the photometer, to analyze the specimen in the reaction container, and to control operations of the discharge mechanism and the water supply mechanism based on the water level of the reaction vessel water detected by the reaction vessel water level detector, characterized in that the reaction vessel water level detector includes a second water level detector for sensing that the water level of the water in the reaction vessel is a measurement limit water level which is lower than the full water level and higher than a position of an optical axis of the light emitted from the light source lamp; wherein the control unit is configured to perform a reaction vessel water replacing operation in which the water in the reaction vessel is drained to the measurement limit water level by controlling the discharging mechanism when the water level of the water in the reaction vessel is the full water level, and the water is supplied to the full water level by controlling the water supply mechanism when the water level of the water in the reaction vessel reaches the measurement limit water level. the water in the reaction vessel is drained to the measurement limit water level by controlling the discharging mechanism when the water level of the water in the reaction vessel is the full water level, and the water is supplied to the full water level by controlling the water supply mechanism when the water level of the water in the reaction vessel reaches the measurement limit water level.