This mouthpiece for washing the oral cavity comprises: a main body part which has a "U" shape in a plan view and has a main body space defined therein； an outer protrusion part which surrounds the outer diameter of the main body part in a plan view, protrudes from both a first surface of the main body part and a second surface that is the opposite surface of the first surface, has an outer protrusion part space defined therein and connected to the main body space, and includes outer washing water discharging ports formed passing therethrough from the outer protrusion part space in the inner diameter direction of the main body part； an inner protrusion part which surrounds the inner diameter of the main body part in a plan view, protrudes from the first surface of the main body part and the second surface that is the opposite surface of the first surface, has an inner protrusion part space defined therein and connected to the main body space, and includes inner washing water discharging ports formed passing therethrough from the inner protrusion part space in the outer diameter direction of the main body part and facing the outer washing water discharging ports； water drainage parts which are formed in the first surface and the second surface and in a shape passing through the main body part, and which form passages for moving liquid on the first surface onto the second surface； a water inlet port part which is connected to the main body space, the outer protrusion part space, and the inner protrusion part space, and has a terminal end to which a washing water supply unit for supplying washing water for washing teeth is connected； a waste water passage part which is formed within the main body space, the outer protrusion part space, and the inner protrusion part space while spatially separated from the space, the outer protrusion part space, and the inner protrusion part space； a suction hole part, which is connected to one end of the waste water passage part and has
