A water quality measuring device comprises a measurement cell (10), a sample water supply unit (80), a reagent supply unit (30), an agitating unit (17), a measuring unit (20) and a control unit (90). The sample water supply unit (80) includes an inlet channel (12) configured to supply the measurement cell (10) with sample water and an outlet channel (13) configured to discharge sample water from the measurement cell (10). The inlet channel (12) is connected to a flow channel (2) configured to supply sample water, and includes a filter (13) and an electromagnetic valve (15). The control unit (90) opens the electromagnetic valve (15) for supplying sample water to allow sample water, from which foreign substances are removed by the filter (13), to flow into the measurement cell (10).