Full color printer by sublimation dye thermal transfer system was developed for proof printing. Proof printing contains various kinds of image such as characters, diagrams, pictures, etc. and it requires many contrast steps and high resolution. Especially, gradation technique is required for fine adjustment to obtain printed matter as close as original. We believed that this gradation technique is important. We realized the gradation of 256 contrast steps with constant density difference between any two adjacent steps by controlling heating time of thermal head. High quality proof is realized by our new invention for gradation technique where one contrast step is finely adjusted by 256 sub-steps. The major specifications of our printer include resolution of 11.8dots/mm, printing speed of 10 msec/line, and printing size of 302.8x439 mm.