To provide a water-ozone mixing device suppressing lowering of mixing efficiency of ozone with a simple constitution when mixing ozone with water; a water purifying device preferably improving water quality with a simple constitution when purifying water; or a safely operable water purifying device; an ozone water generation device efficiently producing ozone water with a simple constitution; a gas-liquid mixer surely preventing reverse flow of liquid to a gas supply side; or a check valve surely preventing the reverse flow.

The water-ozone mixing device 2, water purifying device 1 or ozone water generation device 70 is provided with a drain pipe 35 for draining water leaking from an ozone mixer 7 when water leaks from the ozone mixer 7, and a drain valve 36 provided in the drain pipe 35 and preventing flow-in of air in the direction reverse to the water flow direction while supplying ozone to the ozone mixer 7. The water-ozone mixing device 2 is housed in the casing 27, and an arch valve 81 is used for the check valve 17.