To provide a precise and inexpensive measuring device of pulse light propagation time.

This device comprises a light emission part 12 for emitting a pulse light at a prescribed period; a light receiving part 13 for generating a pulse signal Bp on the basis of the light receipt; a phase lock loop circuit 30 for generating a sine wave-like oscillation signal Bs with high frequency on the basis of the pulse signal Bp; a frequency converting circuit 24 for generating a sine wave-like oscillation signal Cs with low frequency from the sine wave-like oscillation signal Bs by a signal processing for keeping the phase state; a reference signal generating circuit 40 for generating a reference signal Ds equal in frequency to the sine wave-like oscillation signal Cs; and a phase difference detecting means 50 for detecting the phase difference E between the reference signal Ds and the sine wave-like oscillation signal Cs. The pulse light transmission can be reconciled with the phase difference detection by the intervention of the phase lock loop.