PURPOSE: To mix powder in a stable state in a pressurized fluid by fixed quantity by introducing a pressurized fluid increasing in pressure sequentially from each gas introduction port into a cylindrical casing, and from a powder supply port to a discharge port.

CONSTITUTION: A screw feeder comprises a cylindrical casing 1 having a power supply port 2 at one end thereof and a discharge port 4 connected to a transfer path 3 for a pressurized fluid at the other end portion thereof, a screw blade 6 rotatably disposed in the casing 1, and a rotation driving device for rotating the blade 6. The outside diameter of the blade 6 is gradually reduced from one end side to the other end side, and the casing 1 is provided with a plurality of gas introduction ports 7AW7E disposed along the axis thereof. Gas introduction paths 10AW10E for introducing a part of the above pressurized gas are provided at the respective gas introduction ports. Further, pressure reducing valves 11AW11E are respectively interposed in the gas introduction paths, and the set pressure of these pressure reducing valves is gradually decreased from the other end side to one end side.