PURPOSE: To measure the very small amount of ammonia-state nitrogen and total nitrogen precisely in a short time by combining a distilling part and measuring part which are not affected by water.

CONSTITUTION: A specified amount of sample water is introduced into a distilling tank 12 from a sample introducing port 11 of the distilling part 2. Heating is performed by a heater 13 and the ammonia-state nitrogen is transformed to NH3. NH3 is introduced to an electrolytic bath 20 in a coulometric titration part 18 together with carrier gas introduced from an introducing pipe 10 via returning condenser 6 and pipes 5 and 16. NH3 is absorbed by electrolyte. Coulometric titration is performed, the titrated result is displayed on a coulometric titration control device 7, and ammonia-state nitrogen is measured. Then a specifid amount of sample water is injected into a sample introducing port 4 in a measuring part 1, introduced into a reacting part 26 contained in a heating part 25, together with H2 gas which is introduced from an introducing port 27. All nitrogen compound is reduced to NH3. Then NH3 is introduced into an acidic gas removing part 23, coexisting acidic agas is removed, NH3 is introduced into the bath 20 via pipes 21 and 19, coulometric titration is performed, and nitrogen compound is measured.