PURPOSE: To form the same code string conversion patterns by separating digital code strings to the same code strings and alternately magnetizing these code strings to magnetization directions of opposite directions with each of the respective same code strings.

CONSTITUTION: First and second clock signals read out by rotating patterns 1 of the clocks alternately magnetized in the opposite directions shown by arrows of a magnetic disk are shown by 2, 3. Writing is executed by first separating the digital code strings of 4 to the respective same code strings shown by 5 to form writing signals 6 by the two pulses when the two pulses are made correspondent to the respective same code string and the three pulses are made correspondent thereto by as much as the number of their codes, i.e., 1, 2, 1, 1, 3, respectively. The directions of the writing currents are determined for each of the codes 1, 0 of the same code strings of 5 and the currents 7) inverted in their directions by each of the writing signals are passed to a magnetic head, by which the same code string conversion patterns 8 alternately magnetizing 5 in the opposite directions shown by arrows are formed.