A friction stir welding method which permits advantageous production of a product which has a sound welded area without joint defects, by joining together two members formed of the same material or respective different materials selected from a cast aluminum alloy and 2000 series, 4000 series, 5000 series and 7000 series aluminum alloys which are considered difficult to be joined together by the friction stir welding method. An initiation tab member formed of a 1000 series, 3000 series, 6000 series or 8000 series aluminum alloy is disposed in abutting contact with end faces of joining portions of first and second members (in an abutting part), on the side of initiation of the welding operation, and the friction stir welding operation is performed by moving a probe of a rotary tool through the initiation tab member into the joining portions of the two members.