PURPOSE: To form a selector for a chart disc which can position exactly the chart disc by adding a backward rotating mechanism and a mechanism for preventing backward rotation to a servomotor which rotates the chart disc.

CONSTITUTION: A chart disc 4 disposed with plural charts 7, 7' is rotated forward by a servomotor 1. When the desired chart 7 passes slightly by the reference position where the chart faces the optical axis (l) of a light source 14, the motor 1 is run backward in a direction A by the output of a photosensor 12. When the chart 7 returns again to the reference position, a spring 20 is engaged with a click stop 19 of a ratchet type and therefore the backward rotation of the disc 4 is forcibly suppressed. The desired chart 7 is thus positioned exactly in the reference position. The selector for the chart disc which can position exactly the chart disc is formed in the above-mentioned way.