Inspection device and inspection method

An inspection device comprising:

• an irradiation unit configured to irradiate a wafer having a first surface and a second surface with a laser beam from a first surface side of the wafer;
• an imaging unit configured to output light having permeability to the wafer and to detect the light propagating through the wafer;
• and
• a control part configured to perform a first process of controlling the 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 based on a signal output from the imaging unit that has detected the light and deriving a thickness of the wafer from the first surface to the second surface based on the derived position of the modified region and set processing conditions,
• wherein, in the second process, the control part derives a position of a virtual image of an end portion of the modified region on a second surface side based on a signal output from the imaging unit that has detected the light and derives a thickness of the wafer based on the position of the virtual image and the processing conditions.