The present invention aims at providing a transmission wavelength characteristics variable optical element which can actively equalize changes of the wavelength characteristics due to temperature changes, and providing a wavelength characteristics variable apparatus, an optical amplifier, an optical transmission system, and a control method of the transmission wavelength characteristics, which use such an optical element. To this end, the transmission wavelength characteristics variable optical element according to the present invention comprises: an light interference section in which for example reflection films are formed on a light incident plane and a light emitting plane of a magneto-optic crystal; a permanent magnet that applies a fixed magnetic field to the magneto-optic crystal; an electromagnet that applies a variable magnetic field in a direction different from that of the fixed magnetic field; and a variable current source that controls the current flowing in the electromagnet, and varies transmission wavelength characteristics which occur due to an interference of light multireflected between the reflection films, by changing a Faraday rotation angle given to the light being propagated within the magneto-optic crystal, corresponding to a synthetic magnetic field formed by the fixed magnetic field and the variable magnetic field.