Disclosed herein is a using method for a test wafer including a test substrate and a metal foil formed on the front side of the test substrate. The using method includes a modified layer forming step of applying a laser beam having a transmission wavelength to the test substrate from the back side of the test wafer in the condition where the focal point of the laser beam is set inside the test substrate, thereby forming a modified layer inside the test substrate, and a damage detecting step of observing the front side of the test wafer after performing the modified layer forming step, thereby detecting damage to the metal foil.