To unnecessitate a degassing cycle by providing a ring like projection on the end face of extrusion system side of a container, and closing a two-split type seal-block simultaneously bringing the ring like projection into close contact with the extrusion system.

A billet 13 is inserted inside a container 1a and pushed-in forcibly by an extrusion system 70. A freely opening and closing two-split type seal block 40R is closed. A seal packing 41 is made in close contact with a ring like projection 80 and the seal block 40R and the projection 80 are sealed. The seal packing 44 is made in close contact with the extrusion system 70, and the seal block 40R and the extrusion system 70 are sealed. The air existing in the degassing space 31 of the clearance between the container 1a and the billet 13 is evacuated from a vent 45 of the seal block 40R. While performing degassing, the extrusion system 70 is advanced. After completing the degassing, the extrusion is executed. The idle time for the degassing is short.