To provide a nuclear reactor output control system which reduces the load on operators, and efficiently obtaining reactor thermal output by properly adjusting the load set value.

A thermal output changing means 45 provides a load set value changing command S5accordingly to the deviation of a thermal output signal S31from a thermal output set value S41. A thermal output control means 46 outputs an output signal S56which is a command for reducing the load set value to a turbine controller 12 when a deviation signal S49, that is a deviation of the thermal output signal S31from the thermal output upper limit S42, becomes larger than the thermal output upper limit S42, and outputs an output signal S56which is a command for increasing the load set value to a turbine controller when a deviation signal S50, that is a deviation of the thermal output signal S31from the thermal output lower limit S43, becomes smaller than the thermal output lower limit S42.It outputs with priority to the thermal power changing means 45 by a priority circuit 48.

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