The polishing apparatus is capable of changing a pH value of slurry to adjust polishing rate and polishing a work piece with high flatness. The polishing apparatus comprises: a pressure vessel; a polishing plate provided in the pressure vessel; a pressing plate pressing a work piece onto the polishing plate; a driving unit relatively moving the polishing plate with respect to the pressing plate so as to polish the work piece; a gas supplying source supplying an alkaline gas or an acid gas into the pressure vessel; a gas discharging section discharging the supplied gas from the pressure vessel; and a slurry supplying unit supplying slurry onto the polishing plate. A pH value of the slurry is adjusted by dissolving the alkaline gas or the acid gas in the slurry.