For measuring the tritium concentration in water, a water monitoring system was developed. The monitoring system consists of a flow-cell detector, a pair of photomultiplier tubes, a circuit unit (including a high-voltage power supply and a coincidence counting module), a water flow pump and a multichannel pulse height analyzer. The flow-cell detector was fabricated using granular CaF2(Eu), which was solid scintillation materials. The performance of the water monitoring system was examined with three water samples containing different tritium concentrations of 10, 50 and 100 Bq/ml, and linearity between the count rate and the tritium concentration was examined. The results suggest that our system reasonably works as a water monitor for measuring low level tritium concentration. This system is the first such real-time monitoring system able to measure tritium concentrations in water continuously flowing through the solid scintillation detector.