beginning point of a margin on the recording medium the polygon mirror 5 having reflecting surfaces a1 through a6 and driven rotationally in the direction of arrow u03c6 deflects an incident laser beam in the direction of arrow u03b8 on one of reflecting surfaces and the deflected beam then is irradiated onto the film f via the scanning lens 6 and half mirror 7 the buffer unit 2 as shown in fig2 comprises a first line buffer memory 21 a second line buffer memory 22 changeover switches 23 and 24 each selecting different one of the first and second line buffer memories from that selected by the other a gradient control circuit 25 which is supplied with data read out of one of the line buffer memories selected by the changeover switch 24 a digital to analog d a converter 26 for converting a digital signal outputted by the gradient control circuit 25 to an analog signal a timing pulse generating circuit 27 a first line buffer memory clock signal selecting circuit 28 and a second line buffer memory clock signal selecting circuit 29 in the buffer unit 2 data corresponding to a single record line on the recording medium which is readout from the frame memory is written in one of the line buffer memories selected by the changeover switch 23 and during the same time period data.