PURPOSE: To contrive to improve the operation starting efficiency by a method wherein lines to supply steam from outside to down comers are provided in order to forcibly produce circulating water flows while warming up boiler water by supplying the steam at the initial raising of boiler water temperature.

CONSTITUTION: When the raising of boiler water temperature is intended, the heat stored in an accumulator 10 in the form of compressed water due to the operation on the previous day is taken out by flashing operation in the form of steam (d) and led to a pipe line 9. At the same time, the solar rays are irradiated over boiler parts 4 by putting the facilities of solar concentrating system into work. The steams (d) pass a down comer header 3 and ascend the boiler parts 4 so as to heat boiler water by the latent heat of condensation while condensing at the boiler parts 4 during ascending. At the same time, because strong circulating water flow is developed by the buoyancy of steam bubbles and consequently the uniform rising of boiler water temperature is resulted, the occurrence of water hammering and generation of hot spots at the initial raising of boiler water temperature can be dodged. The full utilization of the facilities of the solar concentrating system can be possible without increasing the power consumed in a plant. At the same time, the time for raising boiler water temperature can be shortened.