An optical attenuator (8) includes an input port (1), an output port (4), a fixed reflector (2), a movable reflector (3), two detecting means (6, 7) and a driving device (5). The input port includes a first collimator (13) and a filter (10) attached to the first collimator. The output port includes a second collimator (41) and a splitter (42) connected to the second collimator. Input signals are transmitted from an input fiber (11) through the first collimator and then pass through the filter. The signals passing through the filter are directed by the fixed and the movable reflectors to the second collimator. The angular position of the movable reflector, which is driven by the driving device, determines the proportion of the signals. reflected by the reflectors that are received by the second collimator, which determines the size of the output signals transmitted in an output fiber (421).