A warning device comprising: a foldable driving mechanism comprising: a supporting bracket; and a plurality of walking units mounted to a lower side of the supporting bracket, wherein each of the plurality of walking units comprises: a supporting leg rotatably connected to the supporting bracket, the supporting leg being rotatable between a folding position, where the supporting leg is folded to the supporting bracket, and a walking position, where the supporting leg is substantially perpendicular to the supporting bracket; a warning sign mounted to an upper side of the supporting bracket; the warning sign being covered by reflecting materials; a wheel rotatably connected to an end of the supporting leg; and a fastening unit comprising: a tenon part installed to the supporting leg, the tenon part being a tenon pole slidably connected to the supporting leg; a clamping groove coupled with the tenon part, wherein the clamping groove is defined on the supporting bracket, wherein when the supporting leg is rotated to the walking position, the tenon part is locked to the clamping groove; wherein the tenon pole is slidable between a clamping position, where the tenon pole is inserted into the clamping groove, and a releasing position, where the tenon pole is separated from the clamping groove; and a locking member installed to the supporting leg and coupled with the tenon pole; characterized in that when the supporting leg is perpendicular to the supporting bracket, the tenon pole is inserted into the clamping groove and the tenon pole is locked at the clamping position by the locking member to prevent the supporting leg from rotating with the supporting bracket. a supporting bracket; and a plurality of walking units mounted to a lower side of the supporting bracket, wherein each of the plurality of walking units comprises: a supporting leg rotatably connected to the supporting bracket, the supporting leg being rotatable between a folding position, where the supporting leg is folded to the supporting bracket, and a walking position, where the supporting leg is substantially perpendicular to the supporting bracket; a warning sign mounted to an upper side of the supporting bracket; the warning sign being covered by reflecting materials; a wheel rotatably connected to an end of the supporting leg; and a fastening unit comprising: a tenon part installed to the supporting leg, the tenon part being a tenon pole slidably connected to the supporting leg; a clamping groove coupled with the tenon part, wherein the clamping groove is defined on the supporting bracket, wherein when the supporting leg is rotated to the walking position, the tenon part is locked to the clamping groove; wherein the tenon pole is slidable between a clamping position, where the tenon pole is inserted into the clamping groove, and a releasing position, where the tenon pole is separated from the clamping groove; and a locking member installed to the supporting leg and coupled with the tenon pole; characterized in that when the supporting leg is perpendicular to the supporting bracket, the tenon pole is inserted into the clamping groove and the tenon pole is locked at the clamping position by the locking member to prevent the supporting leg from rotating with the supporting bracket. a supporting leg rotatably connected to the supporting bracket, the supporting leg being rotatable between a folding position, where the supporting leg is folded to the supporting bracket, and a walking position, where the supporting leg is substantially perpendicular to the supporting bracket; a warning sign mounted to an upper side of the supporting bracket; the warning sign being covered by reflecting materials; a wheel rotatably connected to an end of the supporting leg; and a fastening unit comprising: a tenon part installed to the supporting leg, the tenon part being a tenon pole slidably connected to the supporting leg; a clamping groove coupled with the tenon part, wherein the clamping groove is defined on the supporting bracket, wherein when the supporting leg is rotated to the walking position, the tenon part is locked to the clamping groove; wherein the tenon pole is slidable between a clamping position, where the tenon pole is inserted into the clamping groove, and a releasing position, where the tenon pole is separated from the clamping groove; and a locking member installed to the supporting leg and coupled with the tenon pole; a tenon part installed to the supporting leg, the tenon part being a tenon pole slidably connected to the supporting leg; a clamping groove coupled with the tenon part, wherein the clamping groove is defined on the supporting bracket, wherein when the supporting leg is rotated to the walking position, the tenon part is locked to the clamping groove; wherein the tenon pole is slidable between a clamping position, where the tenon pole is inserted into the clamping groove, and a releasing position, where the tenon pole is separated from the clamping groove; and a locking member installed to the supporting leg and coupled with the tenon pole;