An image noise reducing method that can properly reduce both block noise and mosquito noise without deterioration of the image quality is provided. The method includes creating a first luminance component image data with the boundaries of the blocks thereof smoothened, creating a second luminance component image data with its entirety smoothened from the first luminance component image data, creating edge image data by subtracting the second luminance component image data from the first luminance component image data, creating corrected edge image data by applying correction to the edge image data under given conditions, and creating third luminance component image data by adding the corrected edge image data to the second luminance component image data. For color-difference component image data, first color-difference component image data with its entirety smoothened is created from the color-difference component image data. The third luminance component image data and the first color-difference component image data are finally outputted.