1. An image processing apparatus generating a virtual viewpoint image from a captured image acquired by an imaging unit having distortion characteristics and vignetting characteristics, based on distortion characteristics and vignetting characteristics of a virtual camera, the image processing apparatus comprising: a first calculation unit configured to execute a first 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 first distortion correction; a second calculation unit configured to calculate ideal coordinates that are not distorted and correspond to a space position captured at the target pixel in the captured image from the coordinates of the target pixel in the virtual viewpoint image after the distortion correction; a third calculation unit configured to execute a second distortion correction on coordinates of the captured image and to calculate real coordinates in the captured image from the ideal coordinates based on the distortion characteristics of the imaging unit; a fourth calculation unit configured to calculate a pixel value of the target pixel based on the real coordinates and the captured image data; and a correction unit configured to correct the pixel value based on the vignetting characteristics of the imaging unit and the vignetting characteristics of the virtual camera.