PURPOSE: To secure removal through the process wherein a bath boiler is filled with a cleaning liquid consisting of a main alkaline agent, an inorganic peroxide and a heavy metal ion, kept stationary and washed by water when the slime therein has swollen and dissolved.

CONSTITUTION: A cleaning liquid prepared by diffusing or dissolving in water a main alkaline agent, an inorganic peroxide and a heavy metal ion until above pH 11 is obtained is supplied through the lower water pipe 3a of a bath boiler until the liquid flows out of the upper water pipe 3b and kept stationary for more than 3 minutes. Thereafter, the slime having dissolved and swollen is let out together wtih waste liquid by supplying water to the boiler 2 through a water supply pipe 5. Thereby, the slime is swelled satisfactorily in the boiler and the removal thereof is promoted by the froth generated by the peroxide so that secure slime removal becomes possible.