PURPOSE:To improve the quality of intermediate tone with low cost by coding information of plural adjacent dots and sending the result. CONSTITUTION:The size of matrix is decreased as 4 X 4 while keeping the data quantity of 1 bit/dot, each bit at each dot is not handled singly and bits of consecutive dots are handled relatingly. A consecutive 4-bit processing circuit 6 is provided, 16-level of 0-15 of the consecutive 4-bit are coded and the lighting time of a laser is controlled according to the code. Thus, the lighting time of 16-level is set within a time writing 4-dit, the consecutive 4-dot offers 15-valued expressions and in repeating the processing 4 times, set of dots of 15X4=60 gradations is formed. That is, the expression of 60 gradation is attained.