An optical fiber characteristics measurement device (1, 2) comprising: a light source configured to emit a laser light modulated by a prescribed modulation frequency; a light inputting means (12, 13, 15, 16) configured to cause the laser light from the light source to enter one end and another end of an optical fiber as continuous light and pulse light, respectively; and a light detector configured to detect light exiting from the optical fiber, and to measure a characteristic of the optical fiber by using a detection result of the light detector, wherein the optical fiber characteristics measurement device comprises: a synchronous detection device (18, 30) configured to separate out, of a detection signal output from the light detector, a detection signal obtained by detecting light in the vicinity of a measurement point set in the optical fiber, and to detect synchronously a detection signal separated out by using a synchronization signal (SY, SY1) having a prescribed period; wherein the synchronous detection device further comprises: an inverter configured to output an inverted signal that is the detection signal output from the light detector, with the polarity thereof inverted; a non-inverter configured to output a non-inverted signal that is the detection signal output from the light detector, with the polarity thereof not inverted; a switching unit (22, 31) configured to separate out by alternately outputting the inverted signal output from the inverter and the non-inverted signal output from the non-inverter at a timing of the output from the light detector of a detection signal obtained by detecting light in the vicinity of the measurement point set in the optical fiber; and a lowpass filter configured to filter a signal output from the switching unit. a light source configured to emit a laser light modulated by a prescribed modulation frequency; a light inputting means (12, 13, 15, 16) configured to cause the laser light from the light source to enter one end and another end of an optical fiber as continuous light and pulse light, respectively; and a light detector configured to detect light exiting from the optical fiber, and to measure a characteristic of the optical fiber by using a detection result of the light detector, a synchronous detection device (18, 30) configured to separate out, of a detection signal output from the light detector, a detection signal obtained by detecting light in the vicinity of a measurement point set in the optical fiber, and to detect synchronously a detection signal separated out by using a synchronization signal (SY, SY1) having a prescribed period; an inverter configured to output an inverted signal that is the detection signal output from the light detector, with the polarity thereof inverted; a non-inverter configured to output a non-inverted signal that is the detection signal output from the light detector, with the polarity thereof not inverted; a switching unit (22, 31) configured to separate out by alternately outputting the inverted signal output from the inverter and the non-inverted signal output from the non-inverter at a timing of the output from the light detector of a detection signal obtained by detecting light in the vicinity of the measurement point set in the optical fiber; and a lowpass filter configured to filter a signal output from the switching unit.