The present invention relates to control of copper contamination to semiconductor substrates upon operation of a heat treatment apparatus which is a semiconductor manufacturing apparatus and which is constructed with quartz products having been contaminated with copper when machined. The quartz product is placed in a heating atmosphere on the stage where it is not still used for a heat treatment for semiconductor substrates. Baking gases including a hydrogen chloride gas and a gas for enhancing activity of the hydrogen chloride gas, for example, an oxygen gas, are then supplied to the quartz product. Consequently, the copper concentration in the region from the surface to the 30 μm depth of the quartz product can be controlled below 20 ppb, preferably below 3 ppb. The baking process may be carried out before or after assembling the quartz product into the heat treatment apparatus.