To provide a BOD(biological oxygen demand)-measuring instrument, which saves the labor for removing the contamination of a flow path, prevents the flow path from being clogged and restricts fluctuations in measurement.

The BOD-measuring instrument 10 is provided with a microbe electrode 2, a flow cell 3 having a liquid inflow port 31 and a liquid outflow port 32 and arranged so that a liquid passing between the inflow and outflow ports comes in touch with the microbe electrode 2, and liquid introduction parts 7, 8, 9 connected to the inflow port 31 of the flow cell 3 for selecting and introducing a standard solution, an inspection water and a cleaning water to the flow cell 3. All or a part of flow passages of the liquid introduction parts 7, 8, 9, for instance, a flow passage of a sampler 85 is constituted of a silver or copper conduit.