PURPOSE: To enhance accuracy in the measurement by making correctable the lag in AGC control for eliminating the error caused by the amplitude difference when the phase difference is measured between upstream and downstream detection signals.

CONSTITUTION: The mass flowmeter comprises section 58 for detecting the amplitude difference between upstream and downstream oscillations, an amplitude control section 59, and a phase measuring section 60 and measures the phase difference through AGC amplification causing no amplitude difference between upstream and downstream oscillations. A processing unit 61 detects the gain control voltage, frequency and ambient temperature of AGC amplifiers 52a, 52b and determines the propagation delay of the AGC amplifier from the detected value with reference to a ROM table 64. The propagation delay is then subtracted from an output from the phase difference measuring section 60 in order to correct the delay thus realizing an accurate detection.