A CMOS image sensor may include an active pixel sensor suitable for generating a pixel signal corresponding to incident light, a reference pixel array suitable for shielding incident light and generate a reference value, a readout circuit suitable for comparing the pixel signal and the reference value with a ramp signal and for removing a first noise of the pixel signal in a common mode, and a pixel signal processing circuit suitable for performing a linear operation on an output signal from the readout circuit and the reference value, and for removing a second noise of the pixel signal in the common mode.