In a signal reading method for a solid-state image pickup apparatus, an estimated photometric value is calculated at the time of preliminary image pickup and compared with a threshold value set beforehand. If the estimated photometric value is smaller than the threshold value, it is then determined that a scene to be pickup up has a narrower dynamic range to execute control reading out signal charges from the main and subregions of the individual photosensitive cell while mixing them together. If the estimated photometric value is not smaller than the threshold value, the signal charges of the main and subregions are read out independently of each other.