To constitute so as to be able to safely work by preventing a trouble due to a human error of worker.

An extendable mast 6 having a length reaching near the top of the core 5 is placed below a refueling machine 4 and to the tip end of the mast 6, an ultrasonic receiver 7 is fixed. to a control rod drive mechanism 8 below the core 5, an ultrasonic receiver 9 is fixed. The ultrasonic receiver 7 is connected to the first operation indication box 11 with the first signal cable 10. The first operation indication box 11 is connected to a computer 13 for refueling machine with a signal output cable 12. The ultrasonic receiver 9 is connected to the second operation indication box 15 with the second signal cable 14. The second operation indication box 15 is connected to the first operation indication box 11 with the third signal cable. By this, position selection is surely done owing to visualization of the electric signal and indicating core map, the map of fuel loading state in the core as core monitoring device is produced and monitoring of criticality prevention and leak prevention of core water can be attained.