A dishwasher comprising: a tub for accommodating an object to be washed; a main arm rotatably provided in the tub in order to spray wash water to the object to be washed; an auxiliary arm rotatably coupled to the main arm in order to spray the wash water to the object to be washed; a fixed gear unit secured in the tub in order to rotatably support the main arm and having teeth formed along an outer circumferential surface thereof; a rotating gear unit 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 unit, the main arm and the auxiliary arm and configured to be reciprocatingly moved in a diameter direction of the fixed gear unit by rotation of the rotating gear unit so as to allow the auxiliary arm to perform a reciprocating rolling movement to a predetermined angle along a predetermined circular arc path; and an auxiliary arm connection unit connected to the main arm in order to support the auxiliary arm so that the auxiliary arm performs a reciprocating rolling movement to a predetermined angle along the circular arc path, wherein the auxiliary arm connection unit includes an extension pipe provided at the main arm in order to guide wash water supplied to the main arm to the auxiliary arm, a shaft extending from the extension pipe in order to support an interior of the auxiliary arm; characterized in that: the auxiliary arm connection unit includes a sealing member protruding from an outer circumferential surface of the extension pipe in order to prevent wash water, introduced into the auxiliary arm, from flowing backward to the extension pipe, and the sealing member includes a support member provided on at least one of an outer circumferential surface of an upper portion of the sealing member and an outer circumferential surface of a lower portion of the sealing member in order to support the interior of the auxiliary arm; the auxiliary arm connection unit further comprising at least one support rib (910, 920, 930, 940) extending from the support member in a longitudinal direction of the extension pipe. a tub for accommodating an object to be washed; a main arm rotatably provided in the tub in order to spray wash water to the object to be washed; an auxiliary arm rotatably coupled to the main arm in order to spray the wash water to the object to be washed; a fixed gear unit secured in the tub in order to rotatably support the main arm and having teeth formed along an outer circumferential surface thereof; a rotating gear unit 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 unit, the main arm and the auxiliary arm and configured to be reciprocatingly moved in a diameter direction of the fixed gear unit by rotation of the rotating gear unit so as to allow the auxiliary arm to perform a reciprocating rolling movement to a predetermined angle along a predetermined circular arc path; and an auxiliary arm connection unit connected to the main arm in order to support the auxiliary arm so that the auxiliary arm performs a reciprocating rolling movement to a predetermined angle along the circular arc path, wherein the auxiliary arm connection unit includes an extension pipe provided at the main arm in order to guide wash water supplied to the main arm to the auxiliary arm, a shaft extending from the extension pipe in order to support an interior of the auxiliary arm; characterized in that: the auxiliary arm connection unit includes a sealing member protruding from an outer circumferential surface of the extension pipe in order to prevent wash water, introduced into the auxiliary arm, from flowing backward to the extension pipe, and the sealing member includes a support member provided on at least one of an outer circumferential surface of an upper portion of the sealing member and an outer circumferential surface of a lower portion of the sealing member in order to support the interior of the auxiliary arm; the auxiliary arm connection unit includes a sealing member protruding from an outer circumferential surface of the extension pipe in order to prevent wash water, introduced into the auxiliary arm, from flowing backward to the extension pipe, and the sealing member includes a support member provided on at least one of an outer circumferential surface of an upper portion of the sealing member and an outer circumferential surface of a lower portion of the sealing member in order to support the interior of the auxiliary arm; the auxiliary arm connection unit further comprising at least one support rib (910, 920, 930, 940) extending from the support member in a longitudinal direction of the extension pipe.