To prevent both slips caused by damage from projections, and slips caused by adhesive force occurring due to excessive smoothing.The heat treating apparatus includes a processing chamber for heat treating wafers and a boat for supporting the wafers in the processing chamber. The boat further includes a wafer holder in contact with the wafer and a main body for supporting the wafer holder. The wafer holder diameter is 63 to 73 percent of the wafer diameter, and the surface roughness Ra of the portion of the wafer holder in contact with the wafer is set from 1 μm to 1,000 μm. The wafer can be supported so that the amount of wafer displacement is minimal and both slips due to damage from projections on the wafer holder surface, and slips due to the adhesive force occurring because of excessive smoothing can be prevented in that state.