An oral cavity washing device includes: a body unit including a tank which stores a washing liquid, a pump from which the washing liquid stored in the tank is discharged, and a case formed by joining a plurality of parts to each other so as to accommodate the pump; a tube connected to the body unit so as to make the discharged washing liquid flow through the tube; and a washing unit connected to the tube so as to discharge the washing liquid. The oral cavity washing device includes a seal structure having at least a first seal portion which seals a joint portion between the plurality of parts on an outer shell of the case. With such a configuration, the body unit of the oral cavity washing device can be efficiently washed.