To provide an FM modulation measurement method and FM modulation measuring device that are easier and more inexpensive since a precision of 5 GHz in a band of 193.5 THz is needed for an FM modulation measurement method of associated technique, namely, a precision of five digits is needed to make settings difficult, and an electronic spectrum analyzer achieving high precision measurement is expensive.

Signal light including an FM-modulated component is branched into two and after one is delayed, both light beams are combined. At this time, one of the branched signal light beams is so delayed that when the frequency of the one of the branched signal light beams reaches the highest value, the frequency of the other light beam reaches the lowest value. The difference between two frequencies corresponding to -3 dB of the maximum input power of the multiplexed signal light is equal to the FM-modulated component. Further, a measurement band is low, so that the FM-modulated component can be easily be measured by an inexpensive electric spectrum analyzer.