PURPOSE: To read both ends of a mark, by reading marks repeatedly formed with the specified distance a part, and detecting rise-up time and breaking time of thus obtained pulse signals.

CONSTITUTION: Sensors 4a, 4b read group of marks 21a, 21b located on a scale 20. Signals from the sensors 4a, 4b are introduced into monomultiplex control 9a, or 9b. The monomultiplex control 9a detects breaking of a signal S2 and feeds out a signal S4 and the monomultiplex control 9b detects breaking of a signal S6 and feeds out a signals 8. By on OR circuit 14, and summing up signal S4 and S8, pulses 9 generated per each end of each mark of the mark group 21b or obtained. Thus, as both ends of the mark are read, even if a width of the mark is doubled, reading can be available without deterioration of the reading accuracy.