A transmission device includes a transponder including an oscillator configured to oscillate a local optical laser, a wavelength controller configured to control an oscillation wavelength of the local optical laser to receive desired input signal light, and a coherent receiver configured to combine and detect the local optical laser and input signal light. A controller is configured to control the oscillation wavelength using a wavelength of the input signal light detected by the coherent receiver, to determine an erroneous connection of a signal line related to the input signal light, when the wavelength of the input signal light is not an expected wavelength.