1. A polishing apparatus comprising: a polishing table for holding a polishing pad having a polishing surface; a motor configured to drive said polishing table; a holding mechanism configured to hold a substrate having an object material to be polished and to press the substrate against the polishing surface; a dresser configured to dress the polishing surface; and a monitoring unit configured to monitor a removal amount of the object material, said monitoring unit being operable to calculate the removal amount of the object material using a model equation containing a variable representing an integrated value of a torque current of said motor when polishing the object material and a variable representing a cumulative operating time of said dresser.