To execute excellent heating process without causing problems such as shrinkage preventing the damage of object in transporting process, in the heating process of the sheet form object such as dried laver transported by a belt conveyor.

A plurality of the steps of conveyor belts 5a, 5b and 5c are arranged in a heating chamber 1, sucking action is generated by negative pressure to the middle stage conveyor belt 5b hung on a reversing roller 7 arranged to the reversing part 13 of a conveying path, the conveyor belt 5a is touched to the part of the roller 7, sheet form objects 2 fed from the carrying in part 3' are conveyed from the lower stage conveyor belt 5a to the middle stage conveyor belt 5b through a reversing part 13 and from the upper stage conveyor belt 5c to a carrying out part 4' through a similar reversing part 13'. The conveyor direction of the sheet form objects 2 are reversed at each of the reversing parts 13 and 13' while sucked by the conveyor belts 5b and 5c on the upper stage side.