A CMOS image sensor includes a photo-transistor capable of performing photo-sensing and active amplification. The photo-transistor is installed to improve low illustration characteristics while maintaining an existing pixel operation. The CMOS image sensor also includes a reset transistor connected to the photo-transistor and adapted to perform a reset function, a drive transistor for acting as a source follower buffer amplifier in response to an output signal from the photo-transistor, and a switching transistor connected to the drive transistor and adapted to perform an addressing function.