Support device meant to bear in supporting fashion on one or more work pieces in connection with friction stir welding, the support device (20, 20') of which comprises: - a holder unit, - a first support wheel carried by the holder unit, which exhibits an outer peripheral surface and which can be rotated relative to the holder unit about a first rotational axis, - a second support wheel carried by the holder unit, which exhibits an outer peripheral surface and which can be rotated relative to the holder unit around a second rotational axis which is parallel to the said first rotational axis, - a support body connected with the holder unit, which is situated between the support wheels (40, 50) and exhibits a support surface turned outward, and - an endless flexible metal belt, which runs in a loop over the outer peripheral surfaces (41, 51) of the support wheels, and is in sliding contact with the support surface of the support device, wherein the support device (20, 20') is shaped to provide support as it bears on one or more work pieces with the outer peripheral surfaces (41, 51) of the support wheels, and with the support surface of the support body resting against the work pieces via the metal belt, characterized in: - that the holder unit comprises a base part and a holder element that is connected so as to pivot with base part, wherein the holder element is adjustable in various rotary positions relative to the base part by rotation of the holder element relative to the base part about a rotational axis which is parallel with the said first and second rotational axes (R1, R2), - that one of the support wheels is supported so as to rotate at the holder element, and - that the support device (20, 20') exhibits a locking device for secure locking of the holder element on the base part in the desired rotary position relative to it. - a holder unit, - a first support wheel carried by the holder unit, which exhibits an outer peripheral surface and which can be rotated relative to the holder unit about a first rotational axis, - a second support wheel carried by the holder unit, which exhibits an outer peripheral surface and which can be rotated relative to the holder unit around a second rotational axis which is parallel to the said first rotational axis, - a support body connected with the holder unit, which is situated between the support wheels (40, 50) and exhibits a support surface turned outward, and - an endless flexible metal belt, which runs in a loop over the outer peripheral surfaces (41, 51) of the support wheels, and is in sliding contact with the support surface of the support device, wherein the support device (20, 20') is shaped to provide support as it bears on one or more work pieces with the outer peripheral surfaces (41, 51) of the support wheels, and with the support surface of the support body resting against the work pieces via the metal belt, - that the holder unit comprises a base part and a holder element that is connected so as to pivot with base part, wherein the holder element is adjustable in various rotary positions relative to the base part by rotation of the holder element relative to the base part about a rotational axis which is parallel with the said first and second rotational axes (R1, R2), - that one of the support wheels is supported so as to rotate at the holder element, and - that the support device (20, 20') exhibits a locking device for secure locking of the holder element on the base part in the desired rotary position relative to it.