PURPOSE: To provide the video signal processing unit which detects it when a black level point or a white level point is in existence at a signal low level portion of a video signal and which detects its range exactly.

CONSTITUTION: The processing unit is provided with a shading correction circuit S multiplying an inputted video signal (a) with a ratio between a sampling video signal and a prescribed input level and outputting the product, clamp circuits 16, 20 clamping a video output signal (e) of the shading correction circuit S at the prescribed input level, gain amplifiers 17, 21 amplifying clamp output signals f, j and comparators 18, 19, 22, 23 binarizing amplified signals g, k at a setting threshold level and outputting the binarized signals. Thus, even to which part scanned by a measured object a black pint or a white point is in existence, a low level portion in response to a ratio of a signal level of the video signal (a) is obtained by the shading correction circuit S and the clamp output signals f, j are amplified by the gain amplifiers 17, 21 to increase the dynamic range for ease of binarization and a black point (or white point) of the measured object 3 is exactly detected.