To automatically and safely treat a waste liq. contg. heavy metals with a simple structure by introducing the waste liq. into a storage bag, closing the bag and gradually pressurizing the bag with a compressing means while controlling the rate.

The waste liq. contg. heavy metals is mixed with specified chemicals in a flocculating and settling device, and a fluffy deposit as the floc of the heavy metals is formed. The waste liq. contg. the deposit is introduced into a storage bag 1 from its upper opening K, and the upper opening edge 2 of the bag 1 is pressed to close the bag. The bag 1 is then gradually compressed by a compressing means B while controlling its rate with a control means. When the bag is gradually pressurized by the compressing means B in this way, the water in the waste liq. is removed naturally and slowly. Accordingly, the clogging of the bag 1 is prevented, the filtration rate is increased, and the recovery ratio of the treated water is raised.