1. An internal crack detecting method for detecting an internal crack in a wafer having a plurality of devices formed on a front side so as to be separated by a plurality of crossing division lines, said internal crack detecting method comprising: a first image recording step of applying near infrared light having a transmission wavelength to a reference wafer having the same configuration as that of said wafer as a target wafer to be subjected to the detection of said internal crack, thereby obtaining a first image of said reference wafer having no internal crack and then recording said first image; a processing step of processing said target wafer after performing said first image recording step; a second image recording step of applying said near infrared light to said target wafer after performing said processing step, thereby obtaining a second image of said target wafer processed and then recording said second image; and an internal crack detecting step of removing the same image information between said first image and said second image from said second image to obtain a residual image after performing said second image recording step, thereby detecting said residual image as said internal crack in said target wafer.