PURPOSE: To enable the analyzing of the cause of abnormality in a nuclear reactor by providing a means of detecting and recording falling order, falling speed and the like of control rods for a moving object at the abnormal situation, namely, in the falling of the control rods.

CONSTITUTION: Normally, a control rod driving shaft 2 penetrates into a winding element 1e, where an inductance difference detecting circuit 6-1 is kept from outputting signals. As control rods begin falling, the inductance difference detecting circuit 6-1 outputs '1' through an A/D conversion circuit 7 in response to their leaving a winding element 1e and then '0' in response to thir leaving a winding element 1a. A memory device 8 memorizes information on the falling of the control rods and records data of the falling time, falling order and the like.