PURPOSE: To control safely, easily and certainly the depth of a detecting head inserted into a reactor core by providing a driving apparatus for ascending and descending a neutron detecting head in a guide tube at the outside of a housing vessel.

CONSTITUTION: The upper part of a long supporting tube 7 is inserted and passed in the reactor core 1 in an atomic reactor pressure vessel 2, and a detecting head 8 freely movable upward and downward in a long guide tube 5 fixed vertically in the vessel 2 is attached at the upper end part. The driving apparatus 10 penetrates the circumferential wall of the housing vessel 3 containing the casing 2 to the downward part of the tube 7 extending to the outer downward of the casing 2, connected to a joining shaft 55 joined from the outside of the vessel 3, and moves the head 8 upward and downward in the tube 5. The apparatus 10 provides a driving source, an operation controller 12 of the driving source, a transmission apparatus box 20 and a movement end position control mechanism 30. By providing the apparatus 10 at the outside of the vessel 3, the inserting and drawing out positions of the head 8 against the core 1 can be controlled safely, easily and certainly through the mechanism 30 in the low radiation region of the outside of the vessel 3.