Object is to enable a directly-modulated semiconductor laser to be applied as a light source of probe light. A transmission unit configured to generate probe light; and a reception unit including a receiver-side optical bandpass filter that extracts a Stokes component of Brillouin backscattered light from backscattered light which is caused by the probe light in a measurement target optical fiber, and a self-delayed heterodyne interferometer that detects a change in a frequency shift amount of the Stokes component as a phase difference are included. The transmission unit includes a directly-modulated light source configured to generate an optical pulse, and a transmitter-side optical bandpass filter provided in a stage following the directly-modulated light source, and configured to transmit wavelength of an ON level of the optical pulse as the probe light, and block wavelength of an OFF level.