PURPOSE: To simplify the piping of the titled apparatus by using low-cost check valves for preventing the spontaneous flow-out of the reagent from the determination cell by the head difference.

CONSTITUTION: The analyzing apparatus is composed of the aeration tank 13 for the regeneration of the reagent, the determination cell 12 for reacting the specimen with the reagent and placed at a higher position than the aeration tank 13, the reagent discharged line 20 connecting the tank 13 with the cell 12, the check valves 21 and 22 allowing the flow of the reagent toward the aeration tank 13 and attached to the line 20, and a pump 11 for metering the discharge and inserted between the check valves 21 and 22. Static air pressure is applied to the valve 21 as back pressure by the air pump 16 through the tank 13. Accordingly, the spontaneous flow-out of the reagent from the cell by the head difference can be prevented by the use of low-cost check valves, and the piping of the apparatus can be remarkably simplified.