To provide an optical attenuator which has such characteristics that radiation propagated in the attenuator is attenuated depending upon the temperature of the attenuator.

The optical attenuator 200 is equipped with an optical attenuator module 210, an optical divider 220, a detector 240, and a controller 250. Input radiation Pi is received by the attenuator module 210, which attenuates the input radiation to generate corresponding attenuated radiation P0. The detector 240 receives a part 0.1P0 of the attenuated radiation to generate a corresponding electric signal T1. The signal T1 is passed through a 1st feedback loop 262 by a control unit 250 to adjusts the attenuation module 210, thereby holding the radiation power of the attenuated radiation at a specific level.