PURPOSE: To reduce the companding amount of the bellows to drive the caloriemeter by expanding the movement of the rod featuring the linear motion from the outside of the vacuum container by means of the link device and via the bellows.

CONSTITUTION: When male screw 5a is turned from the outside of vacuum container 1, bellows 3 contracts and expands via rotary locking metal fittings 9. And at the same time, rod 5 features the linear motion. Rod 5 is linked to rod 16 featuring free rocking via lever 13, link 20 and others, and the linear motion of rod 5 is expanded by link 20 or the like. Thus rod 16 features the circular motion, and accordingly caloriemeter 15 of the neutron incident device has the circular motion in response to the minor companding amount of bellows 3. As a result, the lateral swing of bellows 3 can be prevented, at the same time minimizing the size of container 1.