An oral cavity washing device according to the present disclosure includes: a nozzle having a flow passage structure including a flow passage that includes an inlet port and an outlet port； and a pump to feed washing liquid to the inlet port such that a flow rate of the washing liquid ejected from the outlet port falls within a range of 200 mL/min to 350 mL/min. The flow passage structure includes a first component, a reducer, and a second component. The first component, the reducer, and the second component are configured to generate cavitation in the washing liquid flowing through the flow passage when the flow rate of the washing liquid ejected from the outlet port falls within the range of 200 mL/min to 350 mL/min.
