An imaging device to which an imaging optical system is attachable, includes a correction data generating unit that generates correction data to correct a sensitivity difference of first phase difference detecting pixel cells and second phase difference detecting pixel cells; and a signal correcting unit that corrects at least one of an output signal of the first phase difference detecting pixel cells and an output signal of the second phase difference detecting pixel cells in accordance with the correction data, in which the correction data generating unit calculates two ratios to generate the correction data based on the two ratios.