An automatic analyzer using a reaction vessels of disposable type is provided which is compact in construction and with high accuracy of measurement.The analyzer is comprised with a reaction container which is capable of having a plurality of cuvettes of disposable type set therein, an extracting and injecting unit for injecting a first reagent, a specimen and a second reagent into a disposable cuvette, a light measuring unit for emitting light to the cuvette, and for measuring absorbance thereof and a CPU for producing a calculated value based on outputs of the light measuring unit.The light measuring unit measures absorbance of the first reagent, specimen and second reagent injected into and reacted with each other in a disposable cuvette, and also measures an air blank value representing absorbance of an empty disposable cuvette and a first reagent blank value representing absorbance of a disposable cuvette having first reagent in the cuvette (S104, S106). The CPU compensates the absorbance based on at least one of the air blank value and the first reagent blank value (S112, S113).