PURPOSE: To obtain a phase-difference measuring apparatus by which the phase difference (the very small time difference) between two signals at an identical frequency can be measured with high accuracy by reducing the required number of bits for an A/D converter.

CONSTITUTION: In addition to respective outputs from an upstream-side pickup 1A and a downstream-side pickup 1B for a flowmeter, outputs from an addition circuit 2 which has added the outputs are quantized by a comparator 3, a PLL circuit 4, antialiasing filters 5A to 5C, sample-and-hold circuits 6A to 6C and A/D converters 7A to 7C. Then, only desired signals are extracted by band-pass filters 8A to 8C so as to be complex-Fourier-transformed by DFTs 9A to 9C. Thereby, the phase difference between two signals at an identical frequency such as an upstream-side pickup signal and a downstream-side pickup signal can be detected with high accuracy by reducing the required number of bits for the A/D converters.