In facilities for the automatic assembly of mechanical parts, a parts supply system has a storing section for storing a tray loaded with parts, a conveying mechanism for conveying the tray from the storing section, and a circulation mechanism for circulating a tray carrier to be loaded with the tray along a circulation path which includes at least a tray loading position for receiving the tray conveyed by the conveying mechanism, a pars supply position for allowing a robot to pick up the parts stored in the tray, and a tray discharge position for discharging the tray. The device simplifies the mechanical arrangement, enhances high speed processing, and readily adapts to a change in the kind of parts and an increase or decrease in the number of parts.