A swinging pinion on the side of an upper swing structure is meshed with a ring gear which is provided on and around the inner periphery of an inner ring on the side of a vehicular base carrier. A grease bath is provided on the inner side of the inner ring and under a swing frame. A center joint is located in an opening which is provided in the swing frame at the center of swinging motions of the upper swing structure. The grease bath is formed by bath-forming walls around an central top plateau wall which is connected the center joint on the inner peripheral side. An annular seal member of resilient material is interposed between the top plateau wall of the grease bath and the lower side of the swing frame. The annular seal member is formed in a height which is larger than the width of spacing between the top plateau wall and the swing frame. At one side, the seal member is detachably fixed either on the side of the swing frame or on the side of the top plateau wall section. At the other side, the seal member is provided with an annular sliding portion which is held in sliding contact with the other one of the swing frame and the top plateau wall in an inclined posture in a radially inward direction. <IMAGE>
