PURPOSE: To shortening replacing time of an expander with a mounting section released by supporting an expander to be changed with a change jig sliding to facilitate the movement of the expander in a working within a bag replaced with a helium gas.

CONSTITUTION: A head of an expander 4 is clamped with a bolt 18 on a flange 17 with a shaft 16. The shaft 16 is positioned horizontally or vertically with a support base 19 of the shaft and the support base 19 is fixed with a bolt 20 on a magnetic shield plate 9. After the replacement of a gas, a worker inserts his hand into a PVC bag 21 in a glove 22 from the side of atmospheric air and a bolt 8 is released. Under such a condition, as the shaft 16 is slidden to the left, the head part of the expander 4 can be separated from first and second cylinders 12 and 13 with a displacer used on. Thus, under such a condition, a worker slides the shaft 16 to the right being replaced with the displacer 31 for exchange and the bolt 8 is clamped on a flange 7 to complete working.