[en] The present invention detects abnormality upon control rod blocking, and contributes to the improvement for reliability of the control rod block monitor which outputs control rod blocking prevention signal. That is, the rod block monitor (RBM) monitors the local power at the periphery of control rods selected for blocking by neutron flux signals sent from a plurality of local power range monitors (LPRM) in the reactor, and when abnormality is detected, the control rod block prevention signal is outputted. The RBM comprises (1) calculation sections for transmitting data relative to external equipments including the LPRM, (2) an N port RAM for fetching data between calculation sections and (3) an address counter for instructing a designation of the data written in the N port RAM. With such a constitution, transferring processing and other signal processing can be conducted dispersingly. As a result, the processing speed is improved, and the number of channels enabling to be processed within a system demand period is increased, thereby enabling to attain a complete dual system. (I.S.)