PURPOSE: To obtain reproduced sound of good S/N by forming an opaque part to small sound to the average width relatively narrower than the average width of an opaque part to large sound.

CONSTITUTION: The opaque part of a sound track is formed to the average width relatively narrower than the average sound track when there are no sounds or the sound is small as compared to the time when the sound is strong to width the width of the transparent part. More specifically, a closing plate 18 moves upward to narrow the part where the plate 18 closes an aperture 16A as the instantaneous value of the input voltage of a modulating driving device 30 is smaller. Conversely, the plate 18 moves downward to width the part where the plate 18 closes the aperture 16A as the instantaneous value of the input voltage of the modulator 30 is larger. The S/N or the time when there are no sounds and when the sound is small is improved to a remarkably high value by determining the characteristics of the device 30 in the above- mentioned manner.