will now be explained in detail with reference to the accompanying drawings showing an embodiment thereof in fig3 a reaction container 21 formed of for example stainless steel is removably screwed to a base plate 20 by a plurality of bolts 22 the container 21 has an upper portion provided with a pipe 21a through which gas is supplied from a gas supply device not shown into the container 21 a plurality of pipes 23 for cooling the container 21 are mounted on the outer peripheral surface of the container 21 the base plate 20 has an annular sealing member 51 provided in a surface thereof contacting the reaction container 21 and further has an exhaust pipe 52 for exhausting gas through the base plate 20 the pipe 52 has a flange 52a having a surface thereof provided with an annular sealing member 53 the base plate 20 has an opening 20a at the center and an upper surface thereof provided with an annular cover 54 covering a peripheral part of the opening 20a a hollow shaft 24 extends through the cover 54 and is rotatably supported by a bearing 25 secured to the lower surface of the base plate 20 thus the opening 20a is sealed by the bearing 25 preferably a bearing which seals a magnetic.