In an image pickup apparatus for preventing linearity defect at the time of photographing in a high-sensitivity mode, when processing an image signal produced by a solid-state image pickup device under a predetermined condition, such as photographing in a super high-sensitivity mode, at a high temperature or with a long-time exposure, a signal processor increases a clamp level for clamping the image signal. A noise reducer then executes noise reduction for removing noise of random nature from the image signal before correcting the primary black level for the image signal, and thereafter the black level is corrected.