PURPOSE: To measure alkali metal ion and ammonium ion continuously without reproduction operation by using an eluent where the compound of an aqueous solution which does not interfere with the measurement of alkali metal ion and ammonium ion is added.

CONSTITUTION: A sample is injected from a sample-injection valve 3. A sample loop 6 at this time is as shown in a broken line. A certain amount of sample is stored in the sample loop 6 and a surplus is unloaded. An eluent is sent from a pump 2 to an isolation column 4. When the sample loop 6 of the sample- injection valve 3 is switched as shown by a solid line, the sample which is stored in the sample loop 6 is introduced to the column 4 and a cation is isolated and is detected by a conductivity detector 5. Then, alkaline earth metal ion and a compound which has a complex-forming capacity and does not interfere with the measurement of alkali metal ion and ammonium ion are added to the eluent and the alkaline earth metal ion which is retained in an ion- exchange group of a cation isolation column is isolated from the ion for measurement purpose and is eluted.