A leakage laser beam detecting method includes a coating step of coating the lower surface of a wafer with an oil marker, thereafter, a press-bonding step of press-bonding an adhesive tape to the lower surface of the wafer, thereafter, a modified layer forming step of applying a laser beam having a wavelength that can be transmitted through the wafer to the wafer from the upper surface thereof while making the laser beam be focused at a focused point within the wafer thereby to form modified layers in the wafer, thereafter, a peeling step of peeling off the press-bonded adhesive tape, and a leakage laser beam detecting step of detecting areas of the lower surface where the oil marker has been removed when the press-bonded adhesive tape is peeled off as areas marked by leakage laser beams.