To provide a strain detection device that obtains a moire pattern even on a transparent plate-like member.

The strain detection device has a transparent surface plate 2 on which plate-like transparent objects 16, 17 to be inspected can be mounted. First and second reference grating units 3, 5 are arranged symmetrically above and below the surface plate, and have surface light emitting units 3a, 5a and surface gratings 3b, 5b arranged on light emitting surfaces 3c, 5c of the surface light emitting units respectively. Surface grating image acquisition means 7 for acquiring superposed image data DAT generated by superposing one over the other a reflected image of the surface grating 3b of the first reference grating unit 3 from the objects 16, 17 to be inspected and an image of the surface grating 5b of the second reference grating unit 5 having been transmitted through the surface plate 2 and the objects 16, 17 to be inspected is provided at a position opposite the first reference grating unit 3, and determines whether or not moire fringes are generated based upon the superposed image data DAT to determine whether the objects 16, 17 are strained or warped.