Optical add/drop multiplexer and optical transmission method

An optical add/drop multiplexer comprising:

• a first wavelength selection switch configured to output an optical signal of each wavelength of an inputted first wavelength multiplexed signal while selecting a path for each wavelength;
• a measurement circuit configured to measure optical power of an inputted optical signal;
• a second wavelength selection switch including
• a circuit configured to output an optical signal of each wavelength of the first wavelength multiplexed signal while selecting a path for each wavelength, in place of the first wavelength selection switch when an abnormality occurs in the first wavelength selection switch, and
• a circuit configured to output an inputted second wavelength multiplexed signal for each prescribed wavelength unit to the measurement circuit when the first wavelength selection switch normally operates;
• and
• a transmitter configured to transmit, to a terminal device connected via a transmission path, spectrum data being generated based on a measurement result of the optical power by the measurement circuit.