Provided is a substrate processing apparatus. The substrate processing apparatus comprises a polishing table; a polishing pad disposed on an upper surface of the polishing table; a conditioner including a conditioner head, a disk holder movably coupled to the conditioner head in a vertical direction, and a conditioning disk mounted to the disk holder and in contact with the polishing pad; and a thickness measuring unit of obtaining the thickness of the polishing pad from the relative moving distance of the disk holder with respect to the conditioner head, wherein the information of the relative moving distance is received from sensing unit.