PURPOSE: To set the alarm value of the neutron count value corresponding to the concn. of controlled plutonium by instantaneously and certainly detecting the abnormality of the concn. of plutonium.

CONSTITUTION: The neutron generated from a small amount of Pu contained in a waste extract is detected by a neutron detection part 1 surrounded by a neutron decelerating material. The signal from the neutron detection part 1 is outputted to an alarm generator 3 as a count ratio through a count circuit 2. A calibrating neutron source 3 is received in a neutron source cask 5 during operation. The calibrating neutron source 4 is inserted in the predetermined place of the neutron detection part 1 prior to operation and it is confirmed that said neutron source 3 is set to a predetermined count ratio. The alarm value 6 of a neutron count ratio is set to the alarm generator 3.