To provide a measuring apparatus far OTDR(optical time domain reflectometer) in which an APD(avalanche photodiode) is protected against damage even if an unexpected abnormal light pulse is generated due to fluctuation in the output level or wavelength of an optical pulse caused by environmental fluctuation.

An ATT 7b attenuates the power of an optical pulse 2 incident from an optical fiber 7a appropriately such that the input voltage to an A/D converter 13 does not exceed an upper limit but has a constant level close to the upper limit. A CPU circuit 14a compares a digital signal a data corresponding to a preset reference waveform and it the digital signal has been varied, delivers a correction data to a D/A converter 15. The D/A converter 15 converts a digital signal, i.e., a correction data received from the CPU circuit 14a, into an analog signal being delivered to an output power regulation circuit 16. The output power regulation circuit 16 delivers a current corresponding, in quantity, to the voltage value inputted from the D/A converter 15 to the ATT 7b where attenuation of power of the optical pulse 2 is controlled.