PURPOSE: To check the deformation at heat treatment and to facilitate maintenance work by heat-treating a wafer in the conditions that a heat-resistant core body is inserted inside a boat where a wafer is mounted, and then carrying in/out the wafer boat alone of a heat treatment device.

CONSTITUTION: A boat 10 to hold wafers 10 and a heat-resistant core body 10 are constituted in separate substances, and the core body 10 is constituted so that it can be inserted into the boat 20 only at heat treatment process. The wafer 30 is heat-treated in the conditions that the heat-resistant core body 10 is inserted inside the boat 20 where wafers 30 are mounted, and then only the wafer boat 20 is carried in/out of the heat treatment device 50 in the conditions that the heat-resistant core body 10 is left on the side of the heat treatment device 50. While checking the deformation at heat treatment this way, jig manufacturing work, cleaning work, and maintenance work such as repair and others can be facilitated.