[en] In the present invention, operationability of control rods upon startup of a BWR type reactor is remarkably improved for the operation safety of the reactor. Based on the address signal of the control rod withdrawn by control rod drives, a neutron monitor disposed near the control rod is determined. A channel judging means for the neutron monitor and a display means for displaying the channel judged by the judging means are provided. With such a constitution, if a control rod is withdrawn by an operator's handling, neutron fluxes in the vicinity of the withdrawn control rod exhibit remarkable increasing rate compared with that in other places. Based on the address signal of the control rod to be withdrawn, the channel of the neutron monitor in the vicinity thereof is indicated to a display means. Accordingly, the channel of the neutron monitor forecasted to react at a high sensitivity by the control rod operation is informed to the operator. Accordingly, the operator can easily know the channel to be noted particularly. (I.S.)