PURPOSE: To forcibly wash and fix material to be processed with each other by providing two housings with an annular space and rotating a mixer shaft in the annular space to constitute a screw conveyor, thereby providing respective mixing means with relative speeds different from each other.

CONSTITUTION: The coaxially rotatable second housing 6 is mounted to the inner side of the static first housing 1 and the annular space 8 of the closed space is disposed between these two housings 1 and 6. The mixer shaft 12 which is driven in the same direction and corotationally moves about the axes of the housings 1 and 6 is annularly arranged in the annular space 8. The enveloping surface of the mixer shaft 12 extends to the inner wall and outer wall of the housings 1 and 6 exclusive of a small spacing and is formed as the screw conveyor 13, by which a fitting groove 17 is formed.