PURPOSE: To allow the dissolved oxygen concentration of a reactor water to be rashly lowered, by means of setting up a piping path which introduces the water maintained within the reactor.

CONSTITUTION: A water sending piping 21 for a condenser branching from a water supplying piping 15 up to a condenser 10, and a water sending piping 24 of the reactor water which connects a reactor 18 and the condenser 10, are set up at the recycling system of the reactor water. The condenser 10 is made into a vacuum state by a vacuum pump 26 when the reactor is in starting, by means of degassing under the vacuum state by allowing the condensed water and the supplied water to be recycled within the closed circuit comprising of a condensing pipe 12, the water supplying piping 15, and the water sending piping 21 for the condenser, and when the d.o.c. may be lowered below 50 ppb, the water is allowed to be degassed under the vacuum by allowing the water to be recycled through the circuit of the condenser 10, a low pressure supplying water heater 13, a high pressure supplying water heater 16, the reactor 18, and the water sending piping 24, and if the d.o.c. may be lowered at the several ppb, turbins 9 are started by means of allowing the reactor water temp. to be raised. Accordingly, the stress corrosion cracking of the materials of the reactor system due to the d.o.c. can be prevented.