1. A clamp control method by a clamp circuit, comprising: as left/right-independent optical black horizontal clamp control method, 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.