To miniaturize a coulometer by integrating an electrolytic cell with a manometer.

Carbon dioxide is increased accompanying the reduction of oxygen in a culture cell 1, caused by the respiration of aerobic microorganisms and this formed carbon dioxide is adsorbed by a carbon dioxide adsorbent 12 to lower the pressure in the culture cell 1. Since the culture cell 1 is connected to an electrolytic cell 14 by an air pipe 13, the surface of the liquid in the air pipe 13 rises, and a current flows across electrodes 14a, 14b and a relay switch is turned on by a relay circuit 16, and a constant current flows across electrodes 14a, 14c. As result, a copper sulfate aqueous solution D is electrolyzed due to the flow of the constant current across both electrodes, and oxygen is generated from the electrode 14a to be supplied to the culture cell 1. The amount of oxygen generated can be calculated, on the basis of the quantity of the current flowing across the electrodes 14a, 14c, and the amount of oxygen consumed by aerobic microorganisms in the sample A within the culture cell 1 can be measured.