PURPOSE: To prevent corrosion of a boiler at rest and shorten the time required for a start of operation when it is resumed, in a system comprising a plurality of sets of continuous blow type boilers.

CONSTITUTION: In a system wherein a plurality of sets of continuous blow type boilers 1, 2 are installed, a method of maintaining the casing of a boiler at rest is such that continuous blow water is continuously supplied from the boiler 1 in operation to a lower drum 20 of the boiler 2 at rest and discharged thereafter from an upper drum 23 thereof.