A switching valve unit, comprising: a piston rod, movable in an axial direction; a valve cylinder, having a piston chamber, for containing said piston rod, and keeping said piston rod slidable between a first position on an upper side, a second position lower than said first position in said axial direction, and a halfway position between said first and second positions; a receiving opening defined in an upper end of said piston chamber; a discharge port hole, defined in a lower end of said piston chamber, for communicating with a discharge conduit; a suction port hole, formed in said valve cylinder, disposed near to said receiving opening, for communicating with a suction conduit; an exhaust port hole, formed in said valve cylinder, disposed nearer to said suction port hole than said discharge port hole, for communicating with an exhaust conduit; seal packing, disposed on said piston rod on a side of said exhaust port hole, for keeping said piston rod fluid-tight in relation to a wall of said piston chamber; a relief passage, formed in said wall of said piston chamber, having a larger width than said seal packing, for receiving said seal packing in case said piston rod is in said second position; a first passage; a second passage (72, 74), formed in said piston rod, for connecting said suction port hole with said exhaust port hole in case said piston rod is in said halfway position; a third passage (69, 72, 73) for connecting said discharge port hole with said exhaust port hole by use of said relief passage in case said piston rod is in said second position, and a cap device; characterized in that said piston rod is a single piston rod adapted to change over between said suction conduit, said discharge conduit and said exhaust conduit, that said first passage is formed in said piston rod, for externally opening said exhaust port hole in case said piston rod is in said first position and that said cap device is adapted to associate said piston rod with said valve cylinder, and to set said piston rod in said first and second positions and said halfway position. a piston rod, movable in an axial direction; a valve cylinder, having a piston chamber, for containing said piston rod, and keeping said piston rod slidable between a first position on an upper side, a second position lower than said first position in said axial direction, and a halfway position between said first and second positions; a receiving opening defined in an upper end of said piston chamber; a discharge port hole, defined in a lower end of said piston chamber, for communicating with a discharge conduit; a suction port hole, formed in said valve cylinder, disposed near to said receiving opening, for communicating with a suction conduit; an exhaust port hole, formed in said valve cylinder, disposed nearer to said suction port hole than said discharge port hole, for communicating with an exhaust conduit; seal packing, disposed on said piston rod on a side of said exhaust port hole, for keeping said piston rod fluid-tight in relation to a wall of said piston chamber; a relief passage, formed in said wall of said piston chamber, having a larger width than said seal packing, for receiving said seal packing in case said piston rod is in said second position; a first passage; a second passage (72, 74), formed in said piston rod, for connecting said suction port hole with said exhaust port hole in case said piston rod is in said halfway position; a third passage (69, 72, 73) for connecting said discharge port hole with said exhaust port hole by use of said relief passage in case said piston rod is in said second position, and a cap device; characterized in that said piston rod is a single piston rod adapted to change over between said suction conduit, said discharge conduit and said exhaust conduit, that said first passage is formed in said piston rod, for externally opening said exhaust port hole in case said piston rod is in said first position and that said cap device is adapted to associate said piston rod with said valve cylinder, and to set said piston rod in said first and second positions and said halfway position.