PURPOSE: To make the temp. in a furnace uniform and to reduce the fundamental unit of fuel by providing plural suction ports for waste gases in the bottom part of a soaking pit of one-way firing in the upper part provided with a burner device in the upper part on the side wall of the furnace.

CONSTITUTION: The waste gases of combustion discharged from a burner 3 provided in the upper part on the side wall 2 of a soaking pit 1 flow in the furnace as shown by solid arrows, turn over from the position of approximately half the furnace length and are sucked into plural pieces of the 1st suction ports 4 for waste gases provided in the hearth. These gases flow through the ports 4 then through a gas conduit 5 into a flue 6. If the 2nd suction port 7 for waste gases is provided below the burner 3, part of the combustion gases are discharged through the port 7 then through the flue 6 according to the degree of opening or closing of a damper device 8. Then the inside of the furnace 1 is heated uniformly, the heating time is reduced and the fundamental unit of fuel is reduced.