To obtain a detergent for removing a scaled coating sludge, having excellent detergency and low flammability, and to provide a method for removing the scaled coating sludge not requiring a manual operation by using the detergent.

This aqueous alkali detergent is used for removing a scaled coating sludge from a solid-liquid separator to which the sludge is stuck and contains an alkali agent, an organic amine, an organic solvent and a surfactant. This method for removing the scaled coating sludge from a centrifugal separator to which the sludge is stuck is characterized in that a detergent is supplied from a detergent tank to the centrifugal separator by a pump to carry out a solid-liquid separation, the sludge is recovered, a liquid is returned to the detergent tank, the coating sludge scaled in the circulation system is removed and the aqueous alkali detergent is used as the detergent.