An image pickup apparatus including an image pickup unit for outputting image signals obtained by respectively photoelectrically converting light passing through different exit pupil areas of a focusing optical system, and a setting unit for setting a focus detection area into an image pickup area of the image pickup unit, stores at least one approximate function for calculating correction values of the image signals obtained by the image pickup unit in an approximation manner, obtains an approximate function effective for the correction at least in the focus detection area set by the setting unit on the basis of at least the one stored approximate function to calculate a correction value in accordance with the obtained approximate function; and performs a focus detection by using the image signals obtained by the image pickup unit and the calculated correction values.