PURPOSE: To shorten a height and to reduce the cost of manufacture by a construction wherein pair of furnace lid lifting devices are disposed oppositely on the opposite side parts holding a furnace body between on a truck located in an upper position of the furnace body and the two furnace lid lifting devices are driven synchronously.

CONSTITUTION: In order to open a furnace lid 14 which closes an opening 12 of a furnace body 10, first, motors 66, 66 of lifting devices 22, 22 are started synchronously. When the motors 66, 66 are driven, vertical shafts 62b, 62b of screw jacks 62, 62 are elevated and thereby moving frames 58, 58 in a pair are elevated in parallel along guide rails 56, 56 respectively. Then, each support member 44 comes into contact with the lower side of a flange 14a and lifts the furnace lid 14 by a prescribed height from the upper end of the furnace body 10. When wheels 28, 28 are rotated by driving drive motors 30, 30 synchronously, subsequently, a truck 20 is separated from the upper part of the furnace body 10 along rails 18, 18 and thereby the opening 12 is opened.