This inspection device includes: a laser irradiation unit that irradiates a wafer having a back surface and a front surface with a laser beam from the back surface side of the wafer; an imaging unit that outputs light having permeability to the wafer and detects the light propagating through the wafer; and a control part configured to perform a first process of controlling the laser irradiation unit so that a modified region is formed inside the wafer by irradiating the wafer with the laser beam and a second process of deriving a position of the modified region on the basis of a signal output from the imaging unit that detects the light and deriving a thickness of the wafer on the basis of the derived position of the modified region and a set recipe.