An encoder having a function for detecting noise which may affect an internal signal of the encoder, without using a noise detecting circuit within the encoder, whereby the cost and size thereof may be reduced. The encoder has a detecting part adapted to detect the movement of an electric motor or a driven object; a signal processing circuit adapted to process a detection signal from the detecting part and obtain positional data of the motor or the object; and a transmitting part adapted to transmit the positional data from the signal processing circuit at regular time intervals and a fluctuation component calculated on the basis of the positional data, to external equipment. The signal processing circuit has a fluctuation component detecting circuit adapted to detect a fluctuation comment included in the calculated positional data, and the detected fluctuation component is transmitted to the external controller as an amount of noise.