Disclosed herein is a technique of removing metal contained in a solution through chelation, which makes it possible to preclude undesired aggregation and permit effective aggregation through an aggregating process. A surface active agent containing a hydrophobic group, a hydrophilic group and a chelating group is added to the solution to be processed, and subsequently an aggregating process is carried out. The chelating group chelates metal, the hydrophilic group prevents undesired aggregation, and the hydrophobic group is effectively aggregated in the aggregating process. Various uses are possible by selecting the charge of ligand.