A first piston that presses friction plates is fitted in a second piston in such a manner as to be movable together with the second piston and relatively movable with respect to the second piston. The method includes incrementally forming a first monotonic wrap surface in a first direction and incrementally forming a second wrap surface in a second direction disposed opposite the first direction. In an ink jet recording medium 5 in a sheet form having an front side 51a and a back side 51b, predetermined portions 53a and 53b, in end faces 52a and 52b constituted by putting sheets of the ink jet recording medium 5 in such a manner that the front side 51a and the back side 51b are arranged in a given direction, have been colored. Dedicated voice trunks are installed between each local VoIP gateway and appropriate selective routers. The communication between the inlet reservoir and main reservoir consists of plurality of ports distributed circumferentially each port being provided with a check valve. Alternatively, it is possible to vertically scroll the images of the musical score section by section on the screen.