Provided is a method for converting a first image signal expressed in a first color space into a second image signal expressed in a second color space on a subword parallelism basis. The method includes determining a first image parameter of a second image signal matrix as a function calculated by multiplying conversion coefficients of a first row constituting an image conversion matrix by image parameters constituting a first image signal matrix; determining a second image parameter of the second image signal matrix as a function expressed by at least one of image parameters constituting the first image signal matrix and the determined first image parameter of the second image signal matrix; determining a third image parameter of the second image signal matrix as a function expressed by at least one image parameter unused for determining the second image parameter of the second image signal matrix among the image parameters constituting the first image signal matrix, and by the determined first image parameter of the second image signal matrix; and finally calculating the second image signal matrix by shifting the determined image parameters constituting the second image signal matrix by a predetermined number of bits.