a fatty acid - related polyunsaturated compound can be selectively purified using an industrially operable successive method under moderate conditions . a highly purified fatty acid - related polyunsaturated compound is obtained by a process comprising steps of forming a complex with silver on account of the high degree of unsaturation of the fatty acid - related polyunsaturated compound , isolating the complex from other fats on account of the hydrophilicity of the complex and then applying dissociating procedures to the complex to obtain the fatty acid - related polyunsaturated compound as a target substance on account of the hydrophobicity of the fatty acid - related polyunsaturated compound . as to the procedures for dissociation , addition of a dissociating agent and reduction of silver are disclosed . as to the dissociating agent , various compounds which produce insoluble composition with silver other then water are disclosed . furthermore , an apparatus appropriately used to carry out this method is also disclosed .