A signal processing method for a Coriolis flowmeter including: performing frequency conversion to combine an oscillation frequency to each of two flow rate signals obtained by A/D conversion on input signals of the phase difference and/or the vibration frequency proportional to the Coriolis force acting on the at least one flow tube; measuring a frequency of a composite waveform associated with at least one of the vibration detection sensors; transmitting a control signal based on the measured frequency; controlling so that a sum frequency component or a difference frequency component of a composite component of a composite frequency signal is constant; and measuring phases from a sum signal or a difference signal of each of controlled converted composite frequencies, to thereby obtain a phase difference signal component.