A nuclear power plant controlling system is provided in which the thermal limit can be brought close to the full limit of operation restrictions by automatic control. The system includes a thermal limit monitor, including a receiver configured to receive a first signal, a prospective time deriving unit configured to derive a prospective time for the first signal to arrive at the full limit, a judging unit configured to judge a remaining time to the prospective time, a compensating unit configured to compensate the first signal based on a second signal, a first transmitter configured to transmit a first instruction to vary a rate factor of the first and second signals by synchronizing the compensation, and a second transmitter configured to transmit a second instruction to hold the first or second signal after arriving at the full limit or at a threshold that is just before the full limit.