An optical transmission device, comprising: an optical splitter configured to branch signal light input from a first optical network (NW#1); a variable optical attenuator configured to control an attenuation amount of first branched signal light branched by the optical splitter and output the first branched signal light for a second optical network (NW#2); and a controller configured to detect an amplitude or intensity modulation component of a signal superimposed on the signal light from second branched signal light branched by the optical splitter and control, based on the detected amplitude or intensity modulation component, an attenuation amount of the variable optical attenuator, so that the amplitude or intensity modulation component is suppressed, further comprising: a delay circuit; wherein a delay amount at the delay circuit is set to a delay amount with which the first branched signal light and the drive signal, which are input to the variable optical attenuator, are synchronized; further comprising: a delay control circuit configured to control a delay amount at the delay circuit based on output light of the variable attenuator, so that the synchronization is established; and wherein the delay control circuit comprises: a pulse width detection unit configured to detect pulse width of the amplitude or intensity modulation component, from the output light of the variable attenuator; a memory configured to store a detection result by the pulse width detection unit; and a setting unit configured to read the detection result from the memory and to set the detection result in the delay circuit. an optical splitter configured to branch signal light input from a first optical network (NW#1); a variable optical attenuator configured to control an attenuation amount of first branched signal light branched by the optical splitter and output the first branched signal light for a second optical network (NW#2); and a controller configured to detect an amplitude or intensity modulation component of a signal superimposed on the signal light from second branched signal light branched by the optical splitter and control, based on the detected amplitude or intensity modulation component, an attenuation amount of the variable optical attenuator, so that the amplitude or intensity modulation component is suppressed, further comprising: a delay circuit; wherein a delay amount at the delay circuit is set to a delay amount with which the first branched signal light and the drive signal, which are input to the variable optical attenuator, are synchronized; further comprising: a delay control circuit configured to control a delay amount at the delay circuit based on output light of the variable attenuator, so that the synchronization is established; and wherein the delay control circuit comprises: a pulse width detection unit configured to detect pulse width of the amplitude or intensity modulation component, from the output light of the variable attenuator; a memory configured to store a detection result by the pulse width detection unit; and a setting unit configured to read the detection result from the memory and to set the detection result in the delay circuit. a delay circuit; wherein a delay amount at the delay circuit is set to a delay amount with which the first branched signal light and the drive signal, which are input to the variable optical attenuator, are synchronized; further comprising: a delay control circuit configured to control a delay amount at the delay circuit based on output light of the variable attenuator, so that the synchronization is established; and wherein a delay control circuit configured to control a delay amount at the delay circuit based on output light of the variable attenuator, so that the synchronization is established; and wherein the delay control circuit comprises: a pulse width detection unit configured to detect pulse width of the amplitude or intensity modulation component, from the output light of the variable attenuator; a memory configured to store a detection result by the pulse width detection unit; and a setting unit configured to read the detection result from the memory and to set the detection result in the delay circuit. a pulse width detection unit configured to detect pulse width of the amplitude or intensity modulation component, from the output light of the variable attenuator; a memory configured to store a detection result by the pulse width detection unit; and a setting unit configured to read the detection result from the memory and to set the detection result in the delay circuit.