A cementing device for establishing a bottom in a well bore for formation of a cement plug in the well bore where the cementing device is adapted for transport into the well bore through a transport pipe, the cementing device comprising: - a tool body which has a front end and a rear end, and comprising ∘ at least one bottom element, wherein the at least one bottom element being foldable into a transport position and expandable into an expanded position to form said bottom in the well bore, wherein the at least one bottom element substantially covers the cross-sectional area of the well bore and forms a false bottom in the well bore during the formation of a cement plug, - at least one propulsion device for propulsion of the cementing device by a fluid through the transport pipe, wherein the propulsion device comprising at least one propulsive element extending radially and being a dish-like element substantially covering the cross-sectional area of the transport pipe, characterized in that the tool body comprises: - a core element arranged between the front end and the rear end of the tool body for stiffening of the tool body, - a plurality of flexible ribs to which the at least one bottom element is attached, the ribs being made of a springy metal and attached at a first end to a fastening element that is moveable along the core element of the tool body, and at a second end attached to the front end of the core element, and - a locking mechanism that is capable of locking the fastening element in the transport position and releasing the fastening element for expansion of the at least one bottom element into the expanded position, wherein the at least one propulsion device is detachably attached to the front end of the tool body and/or the rear end of the tool body. - a tool body which has a front end and a rear end, and comprising ∘ at least one bottom element, wherein the at least one bottom element being foldable into a transport position and expandable into an expanded position to form said bottom in the well bore, wherein the at least one bottom element substantially covers the cross-sectional area of the well bore and forms a false bottom in the well bore during the formation of a cement plug, wherein the at least one bottom element being foldable into a transport position and expandable into an expanded position to form said bottom in the well bore, wherein the at least one bottom element substantially covers the cross-sectional area of the well bore and forms a false bottom in the well bore during the formation of a cement plug, - at least one propulsion device for propulsion of the cementing device by a fluid through the transport pipe, wherein the propulsion device comprising at least one propulsive element extending radially and being a dish-like element substantially covering the cross-sectional area of the transport pipe, - a core element arranged between the front end and the rear end of the tool body for stiffening of the tool body, - a plurality of flexible ribs to which the at least one bottom element is attached, the ribs being made of a springy metal and attached at a first end to a fastening element that is moveable along the core element of the tool body, and at a second end attached to the front end of the core element, and - a locking mechanism that is capable of locking the fastening element in the transport position and releasing the fastening element for expansion of the at least one bottom element into the expanded position,