Abstract: A waste paper de-inking system of this invention agitates and mixes waste paper dissolving slurry and treating gas by a multi-stage type vapor-liquid mixer before a floatator treating step, separates and finely pulverizes ink and sticky matters adhered to fiber in the waste paper dissolving slurry and sufficiently absorbs and contains the treating gas in the slurry before reaching the lower most stage to facilitate de-inking operation in later floatators. Further, the high tower multi-stage vapor-liquid mixer can be utilized also as a floatator to reduce a required floor area and required power consumption of the system.Further, high temperature exhaust gas containing SO.sub.2 produced in a factory is utilized as the treating gas to perform various operations, such as desulfurization, heat recovery, bleaching and also purification of a factory environment, thereby a more economical waste paper de-inking system is provided.