An image sensor includes a pixel sensor outputting an analog sampling signal; a sampling unit comparing the sampling signal and a ramp signal, and outputting a comparison signal that is time-axis length information; and a counter counting a length of the comparison signal based on a clock signal and first and second complement control signals. The counter includes an AND gate ANDing the comparison signal and the clock signal; and a counting unit triggered at a falling edge of the AND gate output to output a count value. The counting unit includes a complement operation controller storing an inverted count value that is an inversion of the count value in response to the first complement control signal, and outputting the inverted count value in response to the second complement control signal; and a D-flip-flop that is set or reset depending on the inverted count value, and outputs the count value.