PURPOSE: To output a plot drawing of high accuracy by providing a reference mark for detecting a position to the surface of a recording medium and optically detecting the movement of the reference mark to obtain the moving data of the recording medium during plotting and subjecting the latent image forming operation of a recording head to timing control.

CONSTITUTION: Reference marks 6 for detecting a moving a position are provided to the side margin part other than the plotting area of a recording medium 1 used in an electrostatic recording type plotter at equal intervals. The reference marks 9 are formed by applying printing processing to the recording medium during a manufacturing process. Plotting data is transmitted to the data buffer of the plotter from a computer 10 and the recording medium 1 begins to move by a drive roller 2. The moving data of the recording medium 1 is obtained by detecting the positions of the reference marks 6 printed on the recording medium 1 in a non-contact state by the optical detector 7 arranged on the lateral side of the upper part of a recording head part 3. Therefore, the point at issue such as the slip between the recording medium and an encoder or the timing lag between the recording operation of the recording heat and the movement of the recording medium is solved.