1. An image processing apparatus generating a virtual viewpoint image from a captured image acquired by an imaging unit having distortion characteristics and ambient light amount decrease characteristics, based on distortion characteristics and ambient light amount decrease characteristics of a virtual camera, the image processing apparatus comprising: a first calculation unit configured to execute a distortion correction on coordinates of a target pixel in the virtual viewpoint image based on the distortion characteristics of the virtual camera and to calculate coordinates in the virtual viewpoint image after the distortion correction; a second calculation unit configured to calculate ideal coordinates in the captured image from the coordinates in the virtual viewpoint image after the distortion correction; a third calculation unit configured to calculate real coordinates in the captured image from the ideal coordinates in the captured image based on the distortion characteristics of the imaging unit; a fourth calculation unit configured to calculate a pixel value corresponding to the real coordinates from image data of the virtual viewpoint image; and a correction unit configured to correct the pixel value corresponding to the real coordinates based on the ambient light amount decrease characteristics of the imaging unit and the ambient light amount decrease characteristics of the virtual camera.