To make it possible to hold inside air tightness by closing a clearance between the furnace body of an arc furnace and a furnace cover.

A ring-shaped seal box 17 is formed around an outer frame 4 of a furnace cover 2. A plurality of air cylinders 18 are vertically laid out on the side surface of the seal box 17. The upper end of a hanger 20 penetrated inside the seal box 17 is connected to and supported on a rod of the air cylinders 18 by way of an arm 19. A seal gasket 21 is mounted on the lower end of the hanger 20 so that it may move slidingly and vetically inside the seal box 17 while a skirt-shaped seal cylindrical body 22 is mounted on the lower end of the hanger 20. The hanger 20 is adapted to drop by the air cylinders 18 so that a seal contact member 23 may be pressed against a top board 3 of a furnace body 1, thereby forming an air tight area around the contact surface between the top board 3 and the outer frame 4.