[Problem] To provide a flow-type ultrasonic oral cavity washing device capable of reliably and efficiently washing the oral cavity. [Solution] In a flow-type ultrasonic oral cavity washing device (1), washing fluid (W1) of a washing fluid supply device (2) is supplied to inside a transducer casing (21) through a supply port (26). Ultrasonic waves (S1) are propagated into the washing fluid (W1) from an ultrasonic transducer (22) mounted in a transducer mounting section (23) inside the transducer casing (21). The flow of washing fluid (W1) is collected at the tip portion inside the transducer casing (21), and then output into the oral cavity from an outflow port (27). In the flow-type ultrasonic oral cavity washing device (1), power of 100W or less is supplied to the ultrasonic transducer (22) that is 50mm or less in diameter, and ultrasonic waves (S1) having a frequency of at least 100kHz but less than 3MHz are propagated into the washing fluid (W1).
