An oral cavity washing device includes a nozzle having a flow passage structure and a pump to feed washing liquid to the flow passage structure. The flow passage structure includes: a first component having a first flow passage a second component being disposed downstream of the first component and having a second flow passage that is narrower than the first flow passage a third component being disposed downstream of the second component and having a third flow passage that is broader than the second flow passage a reducer being disposed downstream of the third component and being configured so as to reduce a width of the flow passage and a fourth component being disposed downstream of the reducer and having a fourth flow passage that widens with an advancement in a path from the reducer to the outlet port so as to generate cavitation in the washing liquid.
