PURPOSE: To enable the starting of a boiler to be restarted for a short period by shortening the time required to release the hot-banking.

CONSTITUTION: A water supplying side 4a of a boiler heat collecting part and a boiler water discharging side 4b are made to communicate with each other by a circulation line 20, and a heat collecting part circulating pump 21 having a low capacity is arranged on the circulation line 20, and boiler water is circulated from the high temperature side to the low temperature side by driving the heat collecting part circulating pump 21 during the hot-banking, thereby the temperature of boiler water of the boiler heat collecting part is equalized in the hot-banking of a boiler which is kept in a halt with heat reserved by the heat collecting part of the boiler by shutting off the water supplying side of the boiler and a steam supplying side.