1. An image processing method, comprising: performing noise reduction processing on an image signal obtained by imaging of a subject image, wherein in the noise reduction processing, a weighted average is calculated on a target of the noise reduction processing in conformity with factors set for a noise reduction filter, the weighted average is an average of a pixel value of the target and pixel values of adjacent pixels, the adjacent pixels are pixels included in a pixel block having the target at a center, the pixel block being a matrix composed of pixels, in the noise reduction filter, the factors are set in advance for each pixel in the pixel block, the factors set for pixels arranged in a horizontal direction in the pixel block are less with an increasing distance from a center in the horizontal direction, the factors set for pixels arranged in a vertical direction in the pixel block are equal, and the factors set for two or more columns are other than zero, each of the columns being pixels arranged in the vertical direction in the pixel block, one of the columns including the target.