Disclosed is an adjuvant for use in simultaneous polishing of a cationically charged material and an anionically charged material, which forms an adsorption layer on the cationically charged material in order to increase polishing selectivity of the anionically charged material, wherein the adjuvant comprises a polyelectrolyte salt containing: (a) a mixture of a linear polyelectrolyte having a weight average molecular weight of 2,000˜50,000 with a graft type polyelectrolyte that has a weight average molecular weight of 1,000˜20,000 and comprises a backbone and a side chain; and (b) a basic material. CMP (chemical mechanical polishing) slurry comprising the above adjuvant and abrasive particles is also disclosed. The adjuvant comprising a mixture of a linear polyelectrolyte with a graft type polyelectrolyte makes it possible to increase polishing selectivity as compared to CMP slurry using the linear polyelectrolyte alone, and to obtain a desired range of polishing selectivity by controlling the ratio of the linear polyelectrolyte to the graft type polyelectrolyte.