PURPOSE: To control the pH N dust before being admixed with a heavy metal elution preventing agent in a specified range at all times.

CONSTITUTION: A kneader 3 for kneading dust mixed with a pH regulator and regulating the pH of the dust and a kneader 4 for kneading the pH- regulated dust mixed with a heavy metal elution preventing agent are independently set in a dust treating device. A part of the dust discharged from the kneader 3 is continuously sampled, the pH of the sample is measured in a dissolution tank 12, a control valve 15 is controlled based on the measured value, and the amt. of the pH regulator to be supplied to the kneader 3 from a pH regulator storage tank 6 is controlled. The dissolving water in a dissolution tank is added to the kneader 4 as additive water.