A phase difference measuring apparatus for precisely measuring a phase difference between two input signals having the same frequency sent from, for example, a Coriolis type mass flowmeter. A differential amplifier in the apparatus provides the sum or difference of the two input signals and a multiplexor sequentially selects and outputs to an A/D converter the two input signals and the output of the differential amplifier. The A/D converter converts the output of the multiplexor into digital data and a digital signal processor or a microcomputer calculates the phase difference between the two input signals based on the converted digital data. The apparatus may further comprise a function that an error caused by the difference of the transfer characteristics of a filter and the A/D converter is compensated using a reference signal which is selected from the two input signals.