A silicon substrate processing method for reducing the thickness of an area of a silicon substrate on which a metal layer is formed to implement a semiconductor integrated circuit is disclosed. The method includes: (A) a process which evenly reduces the thickness of the backside of a silicon substrate to an extent where mechanical strength is maintained and the metal layer on the silicon substrate remains intact; (B) a process which detects defects from the backside of the silicon substrate after the process (A); (C) a process which further reduces the thickness of a defect-containing area of the silicon substrate by processing the backside of the silicon substrate; and (D) a process which measures the thickness of the area of the silicon substrate which is reduced in the process (C).