In an apparatus for detecting a movement of an encoding member capable of generating simultaneously two light beams which are spaced with a predetermined distance therebetween and which move in synchronization with the movement of the encoding member, a line sensor faces the encoding member such that the two light beams move alternately on the line sensor. The line sensor has a length shorter than the distance between the two light beams so that the line sensor receives a single light beam and outputs position information corresponding to a moving position of the received light beam on the line sensor. A detector detects the movement of the encoding member on the basis of the position information output by the line sensor.