A gate 3 for opening and closing an opening 2 of a chamber 1 is fixed to a top end of an arm 7 and is swung between a position facing the opening and a retreated position on a side thereof. A base end of the arm 7 is supported by an output shaft 10 of a fluid pressure cylinder 8 driven in parallel with a direction in which the gate comes into contact with and is separated from the opening 2. A cam follower 12 is provided at the base end of the arm 7 and is movable in a guide groove 14 of a machine frame 6. The guide groove comprises an axial-direction groove 14a for moving the gate 3 from a position facing the opening 2 to a position in the vicinity of the opening of the chamber 1, and a slant-periphery direction groove 14b connected thereto for retreating the gate to the side of the opening.