PURPOSE: To intend simplification of inspection and surveillance work of pipe inside and decrement of workers' radiation exposure by inserting a sealing material from an upper opening of an incore monitor guide tube to seal it off and to prevent reactor water leakage and by providing a water proof camera integrally to the tube, as well.

CONSTITUTION: A seal 1 of an incore guide tube is inserted into an incore monitor guide tube 16 from an upper opening to a reactor upper side, a water proof camera 32 is also inserted to a lower side of the seal 1 and a guide of the camera is gas-tightly connected to a sleeve 7. During inspection work of an inside of the guide tube 16 and of an incore monitor housing 15, the seal 1 is inserted into the guide tube 16 to seal an upper part of the guide tube 16 and therewith reactor water leakage is prevented. Furthermore, by providing the camera 32 integrally to the seal, inside of the guide tube 16 and the housing 15 can be watched so that repairing and inspection works may be easily and efficiently carried out in a faily short time, and decrement of workers' radiation exposure can be successfully intended, as well.