A reflow furnace (1) has a decompression chamber (5), and air bubbles contained in a molten solder on a substrate (7) are removed in this decompression chamber (5). The substrate (7) supported on a first conveying rail (8) is fed into the decompression chamber (5) by a first conveyance rod (18). The substrate (7) in the decompression chamber (5) is taken out by a second conveyance rod (32) and is conveyed to the exit of the reflow furnace (1) while being supported on the second conveying rail (9). Before the treatment of substrates (7) having different widths, the width of the first conveying rail (8) and the width of the second conveying rail (9) are adjusted. During this adjustment, the second conveyance rod (32) is displaced in the width direction of the substrate (7), with the second conveyance rod (32) kept in the decompression chamber (5), and in association with this, the spacing between right and left rail portions (12a, 12a) in the decompression chamber (5) is adjusted.