A water quality measurement device comprising: a UV light irradiation portion that has a passage region through which sample water passes and irradiates the sample water with UV light from around the passage region; an inflow pipe that is arranged to allow sample water to flow into the passage region; an outflow pipe that is arranged to allow the sample water oxidized by being irradiated with UV light by the UV light irradiation portion to flow out of the passage region; and a detector that detects a degradation product that has flowed out into the outflow pipe, wherein: the UV light irradiation portion is a twin tube excimer lamp that has an inner tube in which an internal space is formed as the passage region and an outer tube disposed around the inner tube, and emits UV light by generating a discharge between electrodes provided on an inner peripheral surface of the inner tube and an outer peripheral surface of the outer tube; the inflow pipe is arranged to allow sample water to flow into the inner tube from one end; the outflow pipe is arranged to allow the sample water oxidized by being irradiated with UV light in the inner tube to flow out from the other end of the inner tube; the water quality measurement device further comprises a liquid feed controller that is arranged to repeat an operation of allowing sample water to flow into the inner tube from the inflow pipe, irradiating the sample water in the inner tube with UV light by the excimer lamp with the inflow of sample water stopped, and resuming the inflow of sample water into the inner tube from the inflow pipe to send the oxidized sample water to the detector through the outflow pipe; and the liquid feed controller is arranged to stop the inflow of the sample water and start the operation of the excimer lamp and is arranged to stop the operation of the excimer lamp and start the inflow of new sample water when a predetermined time elapses after the start of the operation of the excimer lamp. a UV light irradiation portion that has a passage region through which sample water passes and irradiates the sample water with UV light from around the passage region; an inflow pipe that is arranged to allow sample water to flow into the passage region; an outflow pipe that is arranged to allow the sample water oxidized by being irradiated with UV light by the UV light irradiation portion to flow out of the passage region; and a detector that detects a degradation product that has flowed out into the outflow pipe, wherein: the UV light irradiation portion is a twin tube excimer lamp that has an inner tube in which an internal space is formed as the passage region and an outer tube disposed around the inner tube, and emits UV light by generating a discharge between electrodes provided on an inner peripheral surface of the inner tube and an outer peripheral surface of the outer tube; the inflow pipe is arranged to allow sample water to flow into the inner tube from one end; the outflow pipe is arranged to allow the sample water oxidized by being irradiated with UV light in the inner tube to flow out from the other end of the inner tube; the water quality measurement device further comprises a liquid feed controller that is arranged to repeat an operation of allowing sample water to flow into the inner tube from the inflow pipe, irradiating the sample water in the inner tube with UV light by the excimer lamp with the inflow of sample water stopped, and resuming the inflow of sample water into the inner tube from the inflow pipe to send the oxidized sample water to the detector through the outflow pipe; and the liquid feed controller is arranged to stop the inflow of the sample water and start the operation of the excimer lamp and is arranged to stop the operation of the excimer lamp and start the inflow of new sample water when a predetermined time elapses after the start of the operation of the excimer lamp. the UV light irradiation portion is a twin tube excimer lamp that has an inner tube in which an internal space is formed as the passage region and an outer tube disposed around the inner tube, and emits UV light by generating a discharge between electrodes provided on an inner peripheral surface of the inner tube and an outer peripheral surface of the outer tube; the inflow pipe is arranged to allow sample water to flow into the inner tube from one end; the outflow pipe is arranged to allow the sample water oxidized by being irradiated with UV light in the inner tube to flow out from the other end of the inner tube; the water quality measurement device further comprises a liquid feed controller that is arranged to repeat an operation of allowing sample water to flow into the inner tube from the inflow pipe, irradiating the sample water in the inner tube with UV light by the excimer lamp with the inflow of sample water stopped, and resuming the inflow of sample water into the inner tube from the inflow pipe to send the oxidized sample water to the detector through the outflow pipe; and the liquid feed controller is arranged to stop the inflow of the sample water and start the operation of the excimer lamp and is arranged to stop the operation of the excimer lamp and start the inflow of new sample water when a predetermined time elapses after the start of the operation of the excimer lamp.