A processing unit for a substrate has a vertical thermal processing furnace 4 having a bottom and an opening provided at the bottom. A boat holding substrates in vertical multistairs can be placed on a first lid, and the first lid can open and close the opening of the vertical thermal processing furnace with the boat placed thereon. The processing unit also has a boat-placing portion on which the boat and another boat can be placed and a boat conveying mechanism for conveying the two boats alternatively between the boat-placing portion and the first lid. A second lid hermetically closes the opening of the vertical thermal processing furnace when the first lid opens the opening but no boat passes through the opening. The processing unit can effectively reduce the undesirable influence from the opening when the first lid is taken off from the opening and the boat is conveyed out, and can also reduce the wasted energy by preventing the fall of the temperature in the interior of the thermal processing furnace.