The semiconductor device manufacturing process for producing semiconductors (transistor, IC, LSI or the like) needs a large number of thermal treatments for semiconductor wafers, such as thermal oxidation, diffusion, CVD, annealing or the like. The above-mentioned various thermal treatments are conducted by employing thermal treatment apparatus. The thermal treatment apparatus according to the present invention performs thermal treatments for semiconductor wafers, such as thermal oxidation, diffusion, CVD, annealing or the like, and has a soft landing loader capable of loading and unloading a wafer jig housing therein a plurality of semiconductor wafers into and from a process tube of the thermal treatment apparatus with high reliability and a high thermal efficiency as well as capable of automatic control of the movement of the semiconductor wafers in accordance with thermal treatment conditions.