A set of patches including a patch of interest formed of pixels relative to a pixel of interest and a similar patch thereof is acquired. A noise amount parameter corresponding to each pixel in the patch of interest is estimated based on a pixel value of the each pixel in the patch of interest. A covariance matrix corresponding to the set of patches on the assumption that each pixel has a uniform noise amount parameter is calculated based on the set of patches. Based on the covariance matrix and the noise amount parameter, a covariance matrix corresponding to the noise amount parameter of the each pixel is estimated. Using the estimated covariance matrix, a patch in which noise of at least the patch of interest among the patches included in the set of patches is reduced is generated.