Abstract: A rotary encoder used for measuring the rotational velocity, for example, of a rotating disk provided with reflecting surfaces and non-reflecting surfaces which are formed alternately on its periphery with a pitch l. The encoder includes one output optical fiber and two input optical fibers. All of the fibers have their one end surfaces opposed to the periphery of the disk. The input fibers are attached to the opposite sides of the output fiber in side-by-side relation. The interval between the end surfaces of the input fibers is set equal to l (n.+-.1/4), where n is a positive integer. Light-emitting devices connected to the input fibers produce output waveforms that are 90 degrees out of phase.