A wavelength dispersion measurement method includes generating a plurality of test lights in the first terminal, the wavelengths of which are different from a wavelength of a signal light, multiplexing each test light with the signal light and outputting the multiplexed light to the first transmission path, reconverting each electrical signal after converting each beam into electrical signals, multiplexing each test light with the signal light and outputting the multiplexed light to the second transmission path, reconverting each electrical signal after converting each test light into electrical signals, multiplexing each test light with the signal light and outputting the multiplexed light to the first transmission path, measuring times for each test light to be propagated up to a specified number of go-around, and measuring a change of a wavelength dispersion amount in the paths based on a difference between the measured propagation times of each wavelength.