Abstract: When an input value is a standard value which is smaller than a maximum value (for example, 230), all pixels of a screen matrix is printed with the same output value (for example, 195), when the input value is within the range not more than the standard value, the output value is limited to not more than 195, and number of the pixels is changed so as to express graduation. In the range where the input value exceeds the standard value, the graduation is expressed by changing the output value of each pixel in the range above 195 while printing all the pixels of the screen.