PURPOSE: To obtain a controlling apparatus of the power of a reactor which prevents unnecessary automatic stoppage of the reactor without increasing the burden of an operator and improves the rate of operation of the reactor, by inserting an optimum selected control rod automatically when a primary loop recirculation pump stops.

CONSTITUTION: A controlling apparatus of the power of a reactor is equipped with primary loop recirculation pump stoppage detecting means 11 which detect stoppage of at least one primary loop recirculation pump 22 and with a selected control rod insertion area determining means 12 which determines an area of insertion of a selected control rod on the basis of the power of the reactor and the flow rate of recirculation drive water. Moreover, it is equipped with a control means 13 which computes the logical product of output signals from these two means and outputs a selected control rod insertion signal, a means 33 which conducts beforehand computation for estimation of an optimum control rod to be inserted according to a situation and a means 31 which selects beforehand the control to be inserted on the basis of the result of computation for estimation.