1. A clamp control method by a clamp circuit for left/right-independent optical black horizontal clamp control, the method comprising: providing horizontal optical black regions to the left and right of an entire effective pixel region of a CMOS image sensor of a column-parallel analog-digital conversion method having two left and right regions, the horizontal optical black regions being a left optical black region and a right optical black region; executing a horizontal clamp process independently for the left and right on virtually-defined left and right effective pixel regions by using optical black; and executing a horizontal clamp process using the right optical black region on the right effective pixel region, wherein, (a) the left/right-independent optical black horizontal clamp control includes sequentially performing processes of (1) setting a deviation from a reference value of a lower limit value of a black level of pixels constituting a row as an optical black value of pixels in each of the left and right optical black regions, (2) calculating a sum of optical black values in each row, (3) holding a value averaged by the number of pixels in a row as an optical black value of the row until scanning of a next row, (4) calling this value at scanning of a row in the effective pixel region to perform operation clamp, and (5) applying the value as a lower limit value of a black level of a next row in the effective pixel region; and (b) performing a clamp process on all rows in the left and right effective pixel regions by associating the left effective pixel region with the left optical black region and associating the right effective pixel region with the right optical black region.