PURPOSE: To accurately measure the concn. of nitrite nitrogen, by using an immobilized bacterial column having nitric acid producing bacterial immobilized thereon and ultraviolet absorptiometry.

CONSTITUTION: Sample water to be measured passes through a filter 1 and is fed to a mixing coil 10 by a liquid feed pump 2 to be mixed with the air from an air pump 9 to be fed to a flow cell 11. Ultraviolet rays are allowed to transmit through the flow cell 11 from a light source 14 and a half mirror 15 and ultraviolet absorbancy is measured by a sensor 16. Subsequently, the sample water is fed to an immobilized bacterial column 12 and the nitrite nitrogen in the sample water flows in a flow cell 13 and ultraviolet rays are allowed to transmit through the flow cell 13 from the light source 14 and the mirror 15 and ultraviolet absorbancy is measured by the sensor 17. The difference between the ultraviolet absorbancies respectively measured by the flow cells 11, 13 is calculated by an operation part 18 to accurately measure the concn. of the nitrite nitrogen in the sample water.