A dishwasher comprising: a tub defining a compartment for accommodating an object to be washed; a spray arm including a main arm rotatably provided in the tub so as to spray wash water toward the object to be washed, and an auxiliary arm coupled to the main arm so as to perform a reciprocating movement along a predetermined circular arc path and to spray wash water toward the object to be washed; a fixed gear part secured in the tub so as to rotatably support the main arm and having teeth formed along an outer circumferential surface thereof; a rotating gear part rotatably mounted to the main arm and configured to be rotated in engagement with the teeth of the fixed gear part by rotation of the main arm; a link unit connected to the rotating gear part, the main arm and the auxiliary arm and configured to be moved by rotation of the rotating gear part so as to allow the auxiliary arm to perform a reciprocating movement along the predetermined circular arc path; characterized by: a foreign substance blocking part for containing the fixed gear part and the rotating gear part so as to prevent the fixed gear part and the rotating gear part from being exposed to wash water, wherein the foreign substance blocking part includes one or more protruding ribs protruding from branched portions of the main arm and the auxiliary arm toward the fixed gear part so as to contain the fixed gear part and the rotating gear part. a tub defining a compartment for accommodating an object to be washed; a spray arm including a main arm rotatably provided in the tub so as to spray wash water toward the object to be washed, and an auxiliary arm coupled to the main arm so as to perform a reciprocating movement along a predetermined circular arc path and to spray wash water toward the object to be washed; a fixed gear part secured in the tub so as to rotatably support the main arm and having teeth formed along an outer circumferential surface thereof; a rotating gear part rotatably mounted to the main arm and configured to be rotated in engagement with the teeth of the fixed gear part by rotation of the main arm; a link unit connected to the rotating gear part, the main arm and the auxiliary arm and configured to be moved by rotation of the rotating gear part so as to allow the auxiliary arm to perform a reciprocating movement along the predetermined circular arc path; characterized by: a foreign substance blocking part for containing the fixed gear part and the rotating gear part so as to prevent the fixed gear part and the rotating gear part from being exposed to wash water, wherein the foreign substance blocking part includes one or more protruding ribs protruding from branched portions of the main arm and the auxiliary arm toward the fixed gear part so as to contain the fixed gear part and the rotating gear part. a foreign substance blocking part for containing the fixed gear part and the rotating gear part so as to prevent the fixed gear part and the rotating gear part from being exposed to wash water, wherein the foreign substance blocking part includes one or more protruding ribs protruding from branched portions of the main arm and the auxiliary arm toward the fixed gear part so as to contain the fixed gear part and the rotating gear part.