An apparatus for automatically starting up a nuclear reactor, which causes control rods in the reactor to be withdrawn in the predetermined order according to the signal obtained by adding the heat-up ratio as a compenstory component determined by the reactor pressure to the deviation of the heat-up ratio of the reactor temperature from the predetermined heat-up ratio and which causes the reactor to be subjected to heat-up and pressurization by opening turbine bypass valves when the heat-up ratio exceeds a reference value.
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