PURPOSE: To well and efficiently execute the treatment of the material to be treated by easily carrying out the opening-closing operation of a cap and surely holding this cap at a closing position.

CONSTITUTION: A rotating vessel 1 opened with a port 1a for putting in and out of the material to be treated on its peripheral wall is rotationally driven by drive mechanism 2, 9 to 11. An opening-closing mechanism supports the cap 18 freely oscillatably between a close position where this port 1a for putting in and out is closed and an open position where the port is opened at the rotating vessel 1 via plural connecting rods 19. The mechanism is so constituted that in the close position the cap 18 is attracted toward the rotating vessel 1 by the connecting rods 19 and may be fixed and held in the state that the cap is engaged with the rotating vessel 1 in tight contact therewith. Consequently, the treatment of the material to be treated is well and efficiency carried out.