A valve positioned in a fluid supply path to control opening and closing of the supply path, comprising: a seat; a first channel disposed in the seat; a second channel disposed in the seat and in communication with the first channel; an inflow channel disposed in the seat and in communication with the second channel, and also in communication indirectly with the first channel via the second channel; an outflow channel disposed in the seat and in communication with the first channel (30, and also in communication indirectly with the inflow channel via the first channel and the second channel; a first valve piece disposed movably in the first channel, wherein the first valve piece can be moved between a first closed position and a first opened position, in which when the first valve piece is in the first closed position, the communication between the first channel and the outflow channel is blocked, whereas when the first valve piece is in the first opened position, the first channel and the outflow channel are put in communication with each other; a second valve piece disposed movably in the second channel, wherein the second valve piece can be moved between a second closed position and a second opened position, in which when the second valve piece is in the second closed position, the communication between the inflow channel and the first channel is blocked, whereas when the second valve piece is in the second opened position, the first channel and the inflow channel are put in communication with each other characterized in that the seat is configured to be joined to an extrusion barrel or mixing barrel of a polymer injection machine so as to form a part of a supply path for an external fluid to be supplied into the extrusion barrel or the mixing barrel. a seat; a first channel disposed in the seat; a second channel disposed in the seat and in communication with the first channel; an inflow channel disposed in the seat and in communication with the second channel, and also in communication indirectly with the first channel via the second channel; an outflow channel disposed in the seat and in communication with the first channel (30, and also in communication indirectly with the inflow channel via the first channel and the second channel; a first valve piece disposed movably in the first channel, wherein the first valve piece can be moved between a first closed position and a first opened position, in which when the first valve piece is in the first closed position, the communication between the first channel and the outflow channel is blocked, whereas when the first valve piece is in the first opened position, the first channel and the outflow channel are put in communication with each other; a second valve piece disposed movably in the second channel, wherein the second valve piece can be moved between a second closed position and a second opened position, in which when the second valve piece is in the second closed position, the communication between the inflow channel and the first channel is blocked, whereas when the second valve piece is in the second opened position, the first channel and the inflow channel are put in communication with each other characterized in that the seat is configured to be joined to an extrusion barrel or mixing barrel of a polymer injection machine so as to form a part of a supply path for an external fluid to be supplied into the extrusion barrel or the mixing barrel.