A solid state imaging apparatus having a plurality of unit pixels, each pixel including a photodiode and a first readout transistor connected to the photodiode, a monitor including a monitor resistor, configured to measure a depletion potential of the monitor resistor, and a readout voltage generator configured to generate a readout voltage to be applied to a first control electrode of the first readout transistor based on the depletion potential. The monitor resistor has the same impurity concentration distribution as that of the photodiode.