A method and an apparatus of reducing the mosquito noise in a decoded image by using a noise reduction algorism that does not depend on the compression coding algorism applied to the image to be improved. The decoded compression-coded digital image is divided into blocks. The standard deviation or the "variance coefficient" of luminance values is calculated for each block of the decoded compression-coded digital image. An .epsilon. matrix for the compression-coded digital image is composed so that all components of each block of the .epsilon. matrix is filled with the standard deviations or the variance coefficients for each block. An .epsilon. filter is composed so that its .epsilon. values are expressed by the .epsilon. matrix. The decoded compression-coded digital image is applied to the .epsilon. filter to reduce the mosquito noise that arises during decoding process.