A swing mechanism for a construction machine with a grease bath seal, having an inner ring on the side of a vehicular base carrier relatively rotatably coupled with an outer ring on a swing frame on the side of an upper swing structure of the construction machine, a ring gear provided on the inner periphery of said inner ring and meshed with a swinging pinion on the side of said upper swing structure, a center joint located within an opening provided in said swing frame at a rotational center thereof, and a grease bath located beneath said swing frame and around said center joint and defining an annular grease bath portion around a top plateau wall connected to said center joint, thereby to lubricate meshed portions of said ring gear and said swing pinion, wherein said grease bath seal comprises: an annular seal member having a height larger than a width of a spacing between said swing frame of said upper swing structure and said top plateau wall of said grease bath in a free state and said annular seal member being interposed in a compressed state between said top plateau wall and said swing frame at a position radially outward of said center joint, characterized in that said said annular seal member is constituted by an annular main block having an anchoring end portion predetermined length on the outer peripheral side and a lipped end portion on the inner peripheral side thereof, said lipped end portion having an annular lip portion erected at and along the inner periphery thereof; said lip portion being provided at the top with a sliding end portion extended out in a radially inward direction for pressed sliding contact with said swing frame in the compressed state of the seal member; said swing mechanism further comprises a seal holder for detachably fixing said seal member, wherein said seal holder being in the form of a ring of a staggered shape in section having a raised seal holder portion at a predetermined height from the upper surface of said top plateau wall of said grease bath for inserting said anchored end portion in a compressed state between said holder portion and said plateau wall, and said seal holder having a base portion formed around the outer periphery of said seal holder portion and fixed on said top plateau wall at a position in the proximity of said center joint. an annular seal member having a height larger than a width of a spacing between said swing frame of said upper swing structure and said top plateau wall of said grease bath in a free state and said annular seal member being interposed in a compressed state between said top plateau wall and said swing frame at a position radially outward of said center joint, characterized in that said said annular seal member is constituted by an annular main block having an anchoring end portion predetermined length on the outer peripheral side and a lipped end portion on the inner peripheral side thereof, said lipped end portion having an annular lip portion erected at and along the inner periphery thereof; said lip portion being provided at the top with a sliding end portion extended out in a radially inward direction for pressed sliding contact with said swing frame in the compressed state of the seal member; said swing mechanism further comprises a seal holder for detachably fixing said seal member, wherein said seal holder being in the form of a ring of a staggered shape in section having a raised seal holder portion at a predetermined height from the upper surface of said top plateau wall of said grease bath for inserting said anchored end portion in a compressed state between said holder portion and said plateau wall, and said seal holder having a base portion formed around the outer periphery of said seal holder portion and fixed on said top plateau wall at a position in the proximity of said center joint.