1. An image processing apparatus which generates an extraction image which extracts streak-like fixed pattern noise in a captured image, comprising: an obtaining unit configured to obtain a black image which is captured in a light-shielded state and contains streak-like fixed pattern noise; a deriving unit configured to derive, for each pixel of the black image obtained by said obtaining unit, an AC component in a first direction in a region containing the pixel; a determination unit configured to determine an intensity of the streak in each pixel of the black image in accordance with an absolute value of the AC component in the first direction that has been derived by said deriving unit; a first reduction unit configured to apply, to the black image obtained by said obtaining unit, a first filter for reducing noise in a second direction intersecting the first direction; a second reduction unit configured to apply, to the black image to which the first filter has been applied, a second filter for reducing noise in the first direction; and an output unit configured to generate the extraction image using the black image to which said first reduction unit has applied the first filter, wherein said output unit generates the extraction image, by weighted-adding the black image to which the first filter has been applied and the black image to which the second filter has been applied using a weight of black image based on the intensity of the streak that has been determined by said determination unit, or by applying, to the black image to which the first filter has been applied, the second filter of which the filter coefficient is controlled based on the intensity of the streak that has been determined by said determination unit.