PURPOSE: To completely prevent foreign matter produced by grinding when a screw is tightened from mixing in, by forming a refrigerant inlet and tapped holes made in parallel at both sides of the inlet in a radiation shield and fixing a seal plate with screws.

CONSTITUTION: An inlet 6 and tapped holes 9 made in parallel at both sides of the inlet are formed on the surface of a wall inside the recessed portion of a radiation shield. After a refrigerant is injected, an O-ring 3 is mounted at the inlet a seal plate 8 is fixed and a screw 7 is inserted into the tapped hole 9 and tightened. It is thereby possible to certainly prevent foreign matter produced by grinding from mixing with refrigerant 5, as the inlet 6 is separated from the tapped holes 9. Foreign matter, which may be produced when the screw 7 is tightened, cannot mix with the refrigerant 5.