High speed of 100 μs, Ultra high-speed & stable measurement for a variety of measurement objects. All deficiencies inherent in the conventional PSD sensing method have now been solved. Whereas the PSD method measures position information from the center of gravity of the total light quantity distribution of the light spots connected along each light element, the linear image sensor method measures the peak position values of the light spots themselves. This advancement now makes high-precision measurement possible, regardless of the surface condition of the object whether for metal hairline surface cracks or for non-reflective black rubber.