A substrate polishing apparatus is provided for preventing excessive polishing and insufficient polishing, and enabling a quantitative setting of an additional polishing time. The substrate polishing apparatus comprises a mechanism for polishing a substrate to be polished; a film thickness measuring device for measuring the thickness of a thin film deposited on the substrate; an interface for entering a target thickness for the polished thin film; a storage area for preserving past polishing results; and a processing unit for calculating a polishing time and a polishing rate. The substrate polishing apparatus builds an additional polishing database for storing data acquired from the result of additional polishing in the storage area.