Wavelength dependence correction method in optical variable attenuator

The present invention provides a wavelength dependent compensation method in a variable optical attenuator which can precisely set the amount of attenuation within a wide range of wavelengths, and to provide superior operability. An optical signal 20a is emitted from a white light source 20 incident on a monochromator 21, and under control of a CPU 24, only an optical signal 20b of a specific wavelength is extracted and is made incident on a variable optical attenuator 22; in optical attenuator 22 under control of a CPU 24, optical signal 20b is attenuated; the power of attenuated optical signal 20c is measured by power measurer 23 under control of a CPU 24, producing compensating data; and using the pulse number corresponding to the rotation angle of the motor which activates the variable optical attenuator 22 according to this compensating data, the position of variable optical attenuator 22 is compensated to attain the desired amount of attenuation.