PURPOSE: To preventing the deformation of wafers held in a furnace in a stable condition without a decrease in throughput by supporting wafers alternately on a fixed member and a movable member that moves vertically in the furnace.

CONSTITUTION: A jig is loaded with a plurality of wafers 10, retained by furnace 9 (not shown is figure) and inserted into a furnace. The furnace is heated, and when a predetermined temperature is reached, a drive is operated to raise the movable boat 2. This brings a projection 8a in a groove 8 into contact with the lower surface of the wafer 10. As a result, the wafer is released from the fixed boat 1 and supported by the movable boat 2, and the wafer is carried upward through a groove 5. The movable boat 2 is then moved downward to support the wafer with a projection 5a of the fixed boat 1. This operation is repeated during the heat treatment to change the point of support of the wafer, thereby preventing the wafer, especially in its periphery, from deformation.