PURPOSE: To rapidly move a detector by inserting a cable provided with a detector on its tip and having a circular member fixed to the cable in the vicinity of the detector into a guide tube inserted and sealed into/in a nuclear reactor and injecting pressurized liquid into a fluid tube.

CONSTITUTION: The guide tube 17 is inserted into the center wall 25 of the nuclear reactor and sealed. The cable 16 to be delivered/rewound by a winch 19 is arranged in the guide tube 17, the detector 15 is fixed to the tip of the cable 16 and the circular member 18 to be smoothly moved in the guide tube 17 is fixed in the vicinity of the detector 15. The fluid tube 22 is arranged between the tip of the guide tube 17 and a pressure impressing/reducing device 21. When the pressurized fluid is spouted from the device 21 by the fluid tubes 22, 23 as shown in the drawing, the detector 15 is rapidly advanced, and when the fluid is spouted in the reverse direction against the shown arrow, the detector 15 is rapidly receded. Consequently, the detector 15 can be rapidly advanced or receded and safely inserted.