To efficiently and surely improve the water quality of a large volume of water in a short time.

The water quality improving device improves the water quality by a which high temperature, high pressure, impulse waves, and ultrasonic waves, etc., caused by destruction of air bubbles due to cavitation, is constituted of a screw 3 generating the cavitation by rotating it in underwater, a flow straightening plate 15 for straightening the flow of the water ejected from the screw 3 and also generating the cavitation, and a grid plate 16 for further generating the cavitation with the water flow straightened the flow with the flow straightening plate 15 and ejected. The water quality improving system is constituted of a water circulation mechanism 26 for circulating the water deteriorated the water quality, and a water quality improving device arranged one or plural set of the devices in the water circulation path of the water circulation mechanism.