A medical device comprising: a treatment main unit; a flexible medium conduit connected to the treatment main unit; a treatment handpiece removably connected to the medium conduit and removably connected to the treatment main unit with the medium conduit being interposed, the medium conduit including an outer tube, a supply tube arranged in the outer tube, the supply tube being configured to supply water from the treatment main unit to the treatment handpiece, and an emission tube arranged in the outer tube and connected to the supply tube, the emission tube being configured to emit water from the medium conduit to the treatment main unit; and a valve arrangement configured to allow the water to flow from the treatment main unit either to the supply tube and the treatment handpiece or to the supply tube and back to the treatment main unit via the emission tube, wherein the treatment main unit includes a medium supply portion configured to supply the water to the supply tube, the valve arrangement includes a supply on-off valve arranged in the treatment main unit between the medium supply portion and the supply tube, and a supply tube on-off valve arranged in a portion of connection of the medium conduit to the treatment handpiece and between the treatment handpiece and a connection portion of the supply tube to which the emission tube is connected, the medical device is configured to allow the water to flow from the treatment main unit to the supply tube and via the emission tube back to the treatment main unit by opening the supply on-off valve in the treatment main unit and closing the supply tube on-off valve, the medium supply portion is connected to a source of tap water and the supply tube is configured to supply tap water from the source of tap water. a treatment main unit; a flexible medium conduit connected to the treatment main unit; a treatment handpiece removably connected to the medium conduit and removably connected to the treatment main unit with the medium conduit being interposed, the medium conduit including an outer tube, a supply tube arranged in the outer tube, the supply tube being configured to supply water from the treatment main unit to the treatment handpiece, and an emission tube arranged in the outer tube and connected to the supply tube, the emission tube being configured to emit water from the medium conduit to the treatment main unit; and an outer tube, a supply tube arranged in the outer tube, the supply tube being configured to supply water from the treatment main unit to the treatment handpiece, and an emission tube arranged in the outer tube and connected to the supply tube, the emission tube being configured to emit water from the medium conduit to the treatment main unit; and a valve arrangement configured to allow the water to flow from the treatment main unit either to the supply tube and the treatment handpiece or to the supply tube and back to the treatment main unit via the emission tube, wherein the treatment main unit includes a medium supply portion configured to supply the water to the supply tube, the valve arrangement includes a supply on-off valve arranged in the treatment main unit between the medium supply portion and the supply tube, and a supply tube on-off valve arranged in a portion of connection of the medium conduit to the treatment handpiece and between the treatment handpiece and a connection portion of the supply tube to which the emission tube is connected, a supply on-off valve arranged in the treatment main unit between the medium supply portion and the supply tube, and a supply tube on-off valve arranged in a portion of connection of the medium conduit to the treatment handpiece and between the treatment handpiece and a connection portion of the supply tube to which the emission tube is connected, the medical device is configured to allow the water to flow from the treatment main unit to the supply tube and via the emission tube back to the treatment main unit by opening the supply on-off valve in the treatment main unit and closing the supply tube on-off valve, the medium supply portion is connected to a source of tap water and the supply tube is configured to supply tap water from the source of tap water.