PURPOSE: To improve accuracy of in-core neutron flux distribution and output distribution by comparing measured values of coolant temperature by a measurement measures and calculated values obtained by calculation.

CONSTITUTION: Coolant temperature at in- and outlet of a nuclear reactor, coolant flow rate, coolant flow rate for fuel assembly and coolant temperature at assembly outlet, of a nuclear reactor core 1, are measured by a coolant flow rate-temperature measurement device 2, and then, from those data, reactor output is calculated and to be transferred to a calculation device 3 of output distribution. Using the calculated value of output distribution, coolant temperature at assembly outlet is calculated by a temperature calculation device 4 of coolant at assembly outlet, and then the results thereof and the measured values, are compared by a comparison device 5. Based on the result, correction factor against the output distribution is calculated by a calculation device 6 of correction factor and then the calculated value of output distribution is multiplied by correction element by a calculation device 7 of correction of output distribution. Based on this result, in-core data required by each display value calculation device 8, are calculated and finally are displayed by a display device 9.