A non-destructive detection method includes, after carrying out a preparation step of preparing an inspection apparatus, an image acquisition step of intermittently moving an objective lens by a predetermined distance in a Z-axis direction orthogonal to an X-Y plane to be made closer to a first surface, positioning a focal point to a Z-axis coordinate value where a distance of the focal point extends by a refractive index of a workpiece, acquiring an X-Y plane image of an inside of the workpiece for each of a plurality of Z-axis coordinate values, and then, recording the acquired images in a recording unit, and a modified layer detecting step of detecting a state of a modified layer from the X-Y plane image for each of the plurality of Z-axis coordinate values recorded in the recording unit.