PURPOSE: To form a read line having a narrow width and to reduce power consumption by irradiating the beam of a laser diode on a voice track while converging the beam with an optical system consisting of a convergent concave mirror having a prescribed inclined angle or the like.

CONSTITUTION: An incident beam 9 is forked by reflecting the 1st-order beam 3 from a laser diode 4 by a plane mirror 12. The incident beam 9 is reflected by a convergent concave mirror 12 to become a read beam 40, which passes a circular opening for irradiation 15 to be irradiated on the voice track of a film 16 and is converted into an electric signal by a photocell 34. At this time, the mean inclined angle of the concave mirror 12 is properly adjusted within the range of 5°-85°. Thus, the width of the read line is made to be a prescribed narrow width and also since the beam having a low divergence is used. the energy loss is prevented and power consumption is reduced by using such an optical system.