To provide a wavelength management system which is hardly affected by temperature change, by applying a fixed cavity Fabry-Perot etalon to the formation of equi-interval transmission maximum values for measuring the scanning range of a scanning heterodyne type spectroscope or a scanning filter type spectroscope.

During laser scanning, a processor confirms the time when a Fabry- Perot transmission peak is confirmed with a coherent receiver and the time when an optical signal is detected, and analyzes the data. A first signal which is detected during scanning, with the coherent receiver during, is set as reference. A computer confirms the transmission peak with the coherent receiver and the elapsed time from the detection time of an optical signal to the time of the next detection. This scanning delay induces the change from scanning to scanning by the change of peripheral temperature of the Fabry-Perot. To each channel, the processor confirms the transmission signal detection time and the delay from the Fabry-Perot maximum value, and inserts the elapsed time in the part between the Fabry-Perot maximum values. Thereby the delay is converted to the frequency deviation.