To realize high-definition smooth printing even when a continuous-gradation image is printed by wet printing by offset printing etc.

A multicolor separation device 20 which generates plate making data by separating image data into multiple colors is equipped with a brightness component separating means 21a of separating the image data into brightness components and hue components to generate characteristic plate making data of the brightness component and storing the characteristic plate making data and hue components in a storage means, a storage means 22 of storing the hue components and characteristic plate making data, and a hue separating means 21b of reading the hue components out of the storage means, separating the components by hues to generate characteristic plate making data of the hue components, and storing them in the storage means.