1. An image processing apparatus comprising: at least one processor or at least one circuit; a memory which is coupled to the at least one processor or the at least one circuit and stores instructions which cause the at least one processor or the at least one circuit to perform operation of following units of the image processing apparatus: 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 first reduction unit configured to apply, to the black image obtained by said obtaining unit for each line of the black image in a first direction, a recursive filter which weighted-adds, in a pixel order in the first direction, a pixel value of a pixel of interest and a pixel value of at least one pixel to which the recursive filter has been applied before the pixel of interest in order to reduce noise in the first direction; and a first output unit configured to delete, from the black image to which said first reduction unit has applied the recursive filter, lines in a second direction intersecting the first direction by a group delay of the recursive filter, generate an image having a line count corresponding to the group delay by using a final line in the second direction in the image from which the lines corresponding to the group delay have been deleted, add the generated image to the image from which the lines corresponding to the group delay have been deleted, and output a resultant image.