A piercing machine that performs piercing-rolling or elongation rolling of a material to produce a hollow shell, comprising: a plurality of skewed rolls disposed around a pass line along which the material passes; a plug disposed on the pass line between a plurality of the skewed rolls; a mandrel bar extending rearward of the plug along the pass line from a rear end of the plug; and an outer surface cooling mechanism disposed around the mandrel bar, at a position that is rearward of the plug, wherein: with respect to an outer surface of the hollow shell advancing through a cooling zone which has a specific length in an axial direction of the mandrel bar and is located rearward of the plug, as seen from an advancing direction of the hollow shell, the outer surface cooling mechanism ejects a cooling fluid toward an upper part of the outer surface, a lower part of the outer surface, a left part of the outer surface and a right part of the outer surface to cool the hollow shell inside the cooling zone, characterized by the piercing machine further comprising: a frontward damming mechanism that is disposed around the mandrel bar at a position that is rearward of the plug and is frontward of the outer surface cooling mechanism, wherein: the frontward damming mechanism comprises a mechanism that, when the outer surface cooling mechanism is cooling the hollow shell in the cooling zone by ejecting the cooling fluid toward the upper part of the outer surface, the lower part of the outer surface, the left part of the outer surface and the right part of the outer surface of the hollow shell, dams the cooling fluid from flowing to the upper part of the outer surface, the lower part of the outer surface, the left part of the outer surface and the right part of the outer surface of the hollow shell before the hollow shell enters the cooling zone. a plurality of skewed rolls disposed around a pass line along which the material passes; a plug disposed on the pass line between a plurality of the skewed rolls; a mandrel bar extending rearward of the plug along the pass line from a rear end of the plug; and an outer surface cooling mechanism disposed around the mandrel bar, at a position that is rearward of the plug, wherein: with respect to an outer surface of the hollow shell advancing through a cooling zone which has a specific length in an axial direction of the mandrel bar and is located rearward of the plug, as seen from an advancing direction of the hollow shell, the outer surface cooling mechanism ejects a cooling fluid toward an upper part of the outer surface, a lower part of the outer surface, a left part of the outer surface and a right part of the outer surface to cool the hollow shell inside the cooling zone, characterized by the piercing machine further comprising: a frontward damming mechanism that is disposed around the mandrel bar at a position that is rearward of the plug and is frontward of the outer surface cooling mechanism, wherein: the frontward damming mechanism comprises a mechanism that, when the outer surface cooling mechanism is cooling the hollow shell in the cooling zone by ejecting the cooling fluid toward the upper part of the outer surface, the lower part of the outer surface, the left part of the outer surface and the right part of the outer surface of the hollow shell, dams the cooling fluid from flowing to the upper part of the outer surface, the lower part of the outer surface, the left part of the outer surface and the right part of the outer surface of the hollow shell before the hollow shell enters the cooling zone. with respect to an outer surface of the hollow shell advancing through a cooling zone which has a specific length in an axial direction of the mandrel bar and is located rearward of the plug, as seen from an advancing direction of the hollow shell, the outer surface cooling mechanism ejects a cooling fluid toward an upper part of the outer surface, a lower part of the outer surface, a left part of the outer surface and a right part of the outer surface to cool the hollow shell inside the cooling zone, characterized by the piercing machine further comprising: a frontward damming mechanism that is disposed around the mandrel bar at a position that is rearward of the plug and is frontward of the outer surface cooling mechanism, wherein: the frontward damming mechanism comprises a mechanism that, when the outer surface cooling mechanism is cooling the hollow shell in the cooling zone by ejecting the cooling fluid toward the upper part of the outer surface, the lower part of the outer surface, the left part of the outer surface and the right part of the outer surface of the hollow shell, dams the cooling fluid from flowing to the upper part of the outer surface, the lower part of the outer surface, the left part of the outer surface and the right part of the outer surface of the hollow shell before the hollow shell enters the cooling zone.