The present invention relates a CMOS (Complementary Metal Oxide Semiconductor) image sensor capable of improving dynamic range by using an additional driver transistor. The CMOS image sensor according to the present invention has a pixel array which has a plurality of unit pixels each of which includes a photodiode and a fist transistor to act as a source follower buffer amplifier to amplify photogenerated charges accumulated in the photodiode. Also, the CMOS image sensor includes a second transistor for a buffer amplifier to amplify and output a gate input voltage in the unit pixel, wherein an output signal of the first transistor is applied to a gate of the second.