1. An image processing apparatus that, for a stereoscopic video image formed of a set of an image for left eye and an image for right eye, corrects gradation of a first image, which is one image from among the image for left eye and the image for right eye, and gradation of a second image, which is the other image from among the image for left eye and the image for right eye, the image processing apparatus comprising: a detection unit configured to detect parallax between the first image and second image; an acquisition unit configured to divide at least one image from among the first image and the second image into a plurality of divided regions, and acquire a respective statistic of a pixel value for each divided region; a decision unit configured to decide a respective gradation conversion curve for each divided region by using the statistic corresponding to the respective divided region acquired by the acquisition unit, and to decide a respective gradation conversion curve for each pixel on the basis of at least a respective gradation conversion curve corresponding to a divided region in which the pixel is located; and a correction unit configured to correct the gradation value of each pixel in the first image by using the respective gradation conversion curve corresponding to the pixel decided by the decision unit and to correct the gradation value of each pixel in the second image, wherein the correction unit detects a pixel in the second image corresponding to a pixel in the first image on the basis of the parallax, and corrects a gradation value of the pixel in the first image and a gradation value of the pixel in the second image corresponding to the pixel in the first image by using a gradation conversion curve based on the respective gradation conversion curve which the decision unit decided for the pixel in the first image.