This invention is achieved to provide a water-flow ultrasonic oral-cavity cleaning device and a method for water-flow ultrasonic oral-cavity cleaning for surely and efficiently cleaning an oral cavity. In the water-flow ultrasonic oral-cavity cleaning device 1, cleaning liquid W1 from a cleaning-liquid supplying device 2 is supplied into a transducer case 21 through a supply port 26. Ultrasonic waves S1 are propagated into the cleaning liquid W1 from an ultrasonic transducer 22 mounted in a transducer mounting part 23 inside the transducer case 21. The water-flow of the cleaning liquid W1 is collected at the apical-part inside the transducer case 21, and then emitted from the issue 27 into the oral cavity, wherein power supply of 100W or less is supplied to the ultrasonic transducer 22 of 50 mm or less in diameter to propagate the ultrasonic waves S1 of 100 kHz or more to 3 MHz or less in frequency into the cleaning liquid W1.
