When clamping a signal from a solid state image sensor, float of an optical black pixel output due to incoming of infrared light avoids a malfunction of a clamp from occurring. When clamping a signal from the solid state image sensor, the difference between the optical black pixel output and a clamp target level is output as a difference output, the difference output is compared with a comparison level to integrate the number of times larger than the comparison level every horizontal line. When the number of times is equal to or more than a certain rate (⅔) from the number of optical black pixels on the horizontal line, an optical black float state is determined and clamping operation is performed in accordance with a held value immediately before.