An automatic analysis device comprising: a light source (160; 17) configured to irradiate a reaction solution with light, wherein the reaction solution contains latex particles of a known size of about 0.1 to 0.3 μm; a first light receiver (161; 22a) and a second light receiver (162; 22b) configured to receive scattered light (21a, 21b) generated from the reaction solution at different light-receiving angles from the irradiation direction of light that irradiates the reaction solution, wherein the light-receiving angle of the first light receiver (161; 22a) is set at 20° and the light-receiving angle of the second light receiver (162; 22b) is set at 30°; a first data processing unit configured to process reaction process data measured by one of the light receivers (161, 162; 22a, 22b) to quantitatively determine a concentration of a substance in the reaction solution; characterised by a second data processing unit configured to determine if the quantitative determination of the concentration of the substance has been performed normally on the basis of whether a ratio of two computed values satisfies a predetermined condition, the two computed values having been calculated from reaction process data measured by the respective first and second light receivers (161, 162; 22a, 22b), wherein the predetermined condition is set such that said ratio of computed values must be about 0.9 to 0.5 based on a known dependency of said ratio of computed values from the particle sizes of latex particles varying within a range of 0.1 to 0.3 μm, so that the second data processing unit is configured to determine that the reaction solution contains foreign matter of a size larger than said known size of the latex particles, when the predetermined condition is not fulfilled because said ratio of computed values is less than 0.5. a light source (160; 17) configured to irradiate a reaction solution with light, wherein the reaction solution contains latex particles of a known size of about 0.1 to 0.3 μm; a first light receiver (161; 22a) and a second light receiver (162; 22b) configured to receive scattered light (21a, 21b) generated from the reaction solution at different light-receiving angles from the irradiation direction of light that irradiates the reaction solution, wherein the light-receiving angle of the first light receiver (161; 22a) is set at 20° and the light-receiving angle of the second light receiver (162; 22b) is set at 30°; a first data processing unit configured to process reaction process data measured by one of the light receivers (161, 162; 22a, 22b) to quantitatively determine a concentration of a substance in the reaction solution; characterised by a second data processing unit configured to determine if the quantitative determination of the concentration of the substance has been performed normally on the basis of whether a ratio of two computed values satisfies a predetermined condition, the two computed values having been calculated from reaction process data measured by the respective first and second light receivers (161, 162; 22a, 22b), wherein the predetermined condition is set such that said ratio of computed values must be about 0.9 to 0.5 based on a known dependency of said ratio of computed values from the particle sizes of latex particles varying within a range of 0.1 to 0.3 μm, so that the second data processing unit is configured to determine that the reaction solution contains foreign matter of a size larger than said known size of the latex particles, when the predetermined condition is not fulfilled because said ratio of computed values is less than 0.5.