The object is to determine a suitable stop timing of dry cleaning operation in a dry cleaning method without an oil-soluble surface-active agent. Contamination level of a pure solvent supplied from a supply tank to an inner drum of a process tank by a supply-side pump is detected by a supply-side RGB sensor. Contamination level of a used solvent collected from the process tank to a distilling unit by a collect-side pump is detected by a collect-side RGB sensor. When the deference of the contamination levels becomes to zero, a control unit stops supplying the pure solvent from the supply tank to the process tank, collecting the used solvent from the process tank to the distilling unit.