An image sensor includes a pixel sensor that senses an incident light and outputs a sampling signal of an analog shape, a sampler that compares the sampling signal and a ramp signal and outputs a comparison signal being time-axis length information, and a gray counter that counts a length of the comparison signal in synchronization with a clock signal and outputs a digital value. The gray counter includes a first flip-flop that divides the clock signal by 2 and generates a first gray code signal, a second flip-flop that delays a first data signal being a four-divided signal of the clock signal and outputs a second gray code signal, and a third flip-flop that delays the second gray code signal being two-divided and outputs a third gray code signal.