[en] Purpose: To enable efficient reactor operation while ensuring sufficient margins for the reactor core stability and the channel stability. Constitution: The stability forecasting device of this invention comprises a reactor core status forecasting device, a stability analyzing device, an operation guide device and a display device. The forecasting device is inputted with each of the status amounts of the reactor core and forecasts the changes of the reactor core power distribution, channel flow rate distribution, reactor core pressure and coolant temperature at the reactor core inlet when predetermined operation procedures are performed. The analyzing device determines the stability amplitude reduction ratio indicating the reactor core stability and the channel stability based on the above-mentioned forecast values. The operation guide device determines the change of the operation for satisfying the operational limit when the stability amplitude reduction ratio deviates from the limited operational value. (Ikeda, J.)