A controlling method of an image sensor is disclosed. The method includes: measuring a first output voltage of a drive transistor, a gate of which is combined to a floating diffusion region, after a predetermined integration time; resetting the floating diffusion region by turning on a reset transistor connected between the floating diffusion region and a power supply group; measuring a reference voltage outputted from the drive transistor; transferring electric charges generated in a photo diode by sensing light inputted from the outside to the floating diffusion region by turning on a transfer transistor connected to the photo diode; and measuring a second output voltage of the drive transistor; wherein an image is generated by using a voltage difference between the first output voltage and the reference voltage, and a voltage difference between the second output voltage and the reference voltage.