Metal scavengers suitable for use in the treatment of waste water are disclosed. The metal scavengers individually comprise a polyamine derivative and a polyethyleneimine derivative. The polyamine derivative is formed of a polyamine having a molecular weight not higher than 500 and contains, per molecule of the polyamine, at least one dithiocarboxyl group or a salt thereof as an N-substituting group substituted for an active hydrogen atom of the polyamine such that at least 30% of suitable active hydrogen atoms contained in the polyamine are substituted by the dithiocarboxyl group or salt thereof. The polyethyleneimine derivative is formed from a polyethyleneimine having an average molecular weight of at least 5,000 and contains, per molecule of the polyethyleneimine, at least one dithiocarboxyl group or a salt thereof as an N-substituting group substituted for an active hydrogen atom of the polyethyleneimine such that at least 40% of substitutable active hydrogen atoms contained in the polyethyleneimine are substituted by the dicarboxyl group or a salt thereof. The mixing ratio of the polyamine derivative to the polyethyleneimine derivative is about 9-7:1-3 by weight.