To provide a device converting a color image from a first color space to a second color space so that the colorimetry of an original image may coincide with its duplicate.

An image processor processes colorimetry values Rc, Gc, and Bc converted from image input device signals Rs, Gs and Bs and generates an address entry with respect to a look-up table storing a pair of conversion coefficient. The colorimetry values Rc, Gc and Bc are converted into coloring agent signals Cx, Mx, Yx and Kx by a conversion coefficient. A value which is not directly mapped from the look-up table is decided by using tetrahedral interpolation for the whole hexagon grids. One of the hexagon grids is generated by shifting every other columns in one dimension by the unit 1/2. As this deviation thicken-packs sample points defining a tetrahedral, a mutual interpolation error is reduced. In addition the thicken-packing facilitates look-up table access to simplify a test for judging which tetrahedral includes a desired value.