A group velocity dispersion measuring device capable of measuring group velocity dispersion (GVD) with a simple configuration and in a short time, comprising an FSF laser (FSFL) (2) for generating frequency chirp light, an optical amplifier (AMP) (3) for amplifying the frequency chirp light, an optical coupler (4), a circulator (5) for branch-controlling optical signals, a photodetector (6) for receiving a ray of light before and after its propagation through an optical fiber (1) to be measured, and an RF spectrum analyzer (RFSA) (7) for observing an optical spectrum waveform. Beat signals generated between chirp lights before and after the propagation, through the optical fiber (1) to be measured, of frequency chirp light emitted from the FSF laser (2) are alternately observed to calculate a GVD value from frequency differences between these beat signals.