An analog-to-digital conversion method may include: generating an initial comparison signal by storing adjacent pixel signals and comparing the adjacent pixel signals, and generating a first control signal based on the generated initial comparison signal; generating a reference comparison signal by comparing the adjacent pixel signals based on the reference signal and a ramp-up signal switched according to the generated first control signal, and determining a ramping direction according to the generated reference comparison signal and generating a second control signal; and performing data conversion by selecting any one of the ramp-up signal and a ramp-down signal according to the generated second control signal, and by comparing the selected ramp signal with a ‘difference value between the adjacent pixel signals’.