For a solid-state image pickup device 1, a plurality of pixels are two dimensionally arranged in an imaging region 10, and two photodiodes PD1 and PD2 are included in each pixel Pm,n. An electric charge generated in the respective photodiodes PD1 and PD2 is input to a signal readout section 20, and a voltage according to an electric charge amount thereof is output from the signal output section 20. The voltage output from the signal readout section 20 is input to an A/D converting section 40, and a digital value according to the input voltage is output from the A/D converting section 40. In an adding section 50, a sum of digital values to be output from the A/D converting section 40 according to the amount of electric charge generated, for each pixel Pm,n of the imaging region 10, in the two respective photodiodes PD1 and PD2 included in the pixel is operated, and a digital value being a sum value thereof is output.