To provide a boiler scale removing method for promoting scale purging to shorten the time required for removing scale and reduce cost even when peeling-off of scale in large scale occurs.

A boiler (a furnace 11a) and a condenser 61 are mutually connected through a bypass passage 91 and steam pressure circulating in a condensation/steam system 120 switched into the bypass passage 91 pulsates (swings) to promote scale purging on the way to the condensation/steam system 120. A pulsation process of steam pressure is composed of a plurality of pulsation stages having differences in steam pressure being different from each other. In the plurality of pulsation stages, difference in steam pressure is gradually increased as the pulsation process of steam pressure advances.

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