The present invention provides an optical transmission device, a transmission system, and a control method for a transmission system which make it possible to adjust the wavelength band of dummy light according to the wavelength band of an added main signal. This optical transmission device comprises: an output branching unit which multiplexes and outputs an added main signal and dummy light; a wavelength adjustment unit which adjusts the wavelength band of the dummy light; a signal detection unit to which an optical signal outputted by the output branching unit is inputted, and which detects the wavelength band of the added main signal and outputs a detection result; and a control unit which controls the wavelength adjustment unit according to the detection result from the signal detection unit.