A surface treatment method for metal parts includes a polishing step of supplying and discharging a cleaning liquid into and from a barrel tub while mass including a metal part and a medium is caused to flow in the barrel tub, so that a surface of the metal part is polished. The polishing step is carried out at least once. The polishing step includes a final finish polishing process in which a final finish medium which is free from abrasive grain or which consists of a synthetic resin base material and is free from abrasive grain or which is made by binding abrasive grain of not more than 10 wt % and a synthetic resin binding material together and is free from alumina is used as the medium.